Why is "cupboard" pronounced with a silent "p"?

There are sev­er­al fac­tors in play here.

Dif­fi­cult con­so­nant clus­ters are of­ten re­duced in rapid speech or over time; think of friend­ship, spend­thrift, twelfth, months.

Much of the dif­fer­ence be­tween an un­voiced and a voiced stop in English is ac­tu­al­ly not its voic­ing but its as­pi­ra­tion, and be­cause one nor­mal­ly on­ly as­pi­rates stops that are both un­voiced and which be­gin a stressed syl­la­ble, you have just lost the prin­ci­pal dis­tin­guish­ing fea­ture.

When you have two con­sec­u­tive stops that dif­fer on­ly in voic­ing, these are es­pe­cial­ly like­ly to fuse, with the first of the pair dropped. Without an au­di­ble re­lease, there is noth­ing to mark the end of one and the be­gin­ning of the next.

Here is a set of words or phras­es where you nor­mal­ly sup­press one of the two ad­ja­cent stops that dif­fer on­ly in voic­ing:

• cup­board
• rasp­ber­ry
• black­guard
• back­ground
• post­doc
• post­dat­ed check
• sub­poe­na
• next-door neigh­bor
• last-ditch ef­fort
• best dog­sit­ter

It is not al­ways the first of the two that is sup­pressed. For ex­am­ple, no­tice how in back­ground noise, it is the g that ap­pears to get lost: it sounds more like back round.

In con­trast, in black­guard (when pro­nounced as though it were spelled blag­gerd) it is the first of the two ad­ja­cent stops that seems to go away, mak­ing it work like cup­board and rasp­ber­ry with their lost p.

A lost dog may well come out sound­ing like a loss dog in rapid speech, and a black glass like a black lass.

This is not com­plete­ly guar­an­teed, es­pe­cial­ly in new com­pounds whose mor­phemic bound­aries are still clear. It is al­so more apt to hap­pen when the stress is on the first syl­la­ble than when it’s on the sec­ond. But on­ly very care­ful speak­ers will gem­i­nate stops when go­ing out­doors: the t be­comes at most a glot­tal stop — if that. So an out­door the­ater might be said [ˌäʊ̯ʔ.doɻʷ ˈθiː.əɾɚ].

But even a big kite, a bad turn, or a job posting is li­able to lose the first of the paired stops in con­nect­ed speech, since the sec­ond stop is as­pi­rat­ed and the first gets no au­di­ble re­lease.

Just to emphasise the pronunciation guides that people have given elsewhere, it's not pronounced as "cup-board" or "cu-board" but really "cubbered" very similar to "covered".

You have to really think of English as 2 separate languages; the spoken one that has dynamically evolved for a thousand years and the written one which was codified 500 years ago into standard spelling. Over time, the pronunciation is going to drift further and further away from the spelling such that the written version of a word will contain virtually no clue as to how it's pronounced - but will just serve as a generally-accepted "code" that we all know and understand.

It has the added benefit that the "code" will be an endless source of fascination for people like us explore how our "cubbered" must have evolved from a board that cups were put on, that gained some sides, then a top, then, finally, some doors.

In short, in Britain today, there really is no "p" in "cubbered" - except in the archaic spelling "code" that we use to represent it. You may not like it but that's the way it is.

There will always be people at the forefront of the spoken evolution - and those lagging behind. It is interesting that we never seem to hear people campaigning for the proper pronunciation of "knife" as "k-neef" as it "should" be said. Even better, "knight" as "k-nichhter". There are countless, no doubt better, examples if I could think of them.

Perhaps the closest spelling match to cupboard is clapboard (which can refer either to a type of exterior wall used in building houses and other structures or to the rectangular device used to designate a scene and take in filming and to signal the beginning of the scene).

According to the phonetic system that Merriam-Webster's Eleventh Collegiate Dictionary uses, cupboard has the pronunciation given in this entry:

cupboard \ˈkə-bərd\ n (1530) : a closet with shelves where dishes, utensils, or food is kept; also, a small closet

In contrast, the pronunciation of clapboard is variable and, to some extent, depends on the sense of the word that one intends. Again from the Eleventh Collegiate:

clapboard \ˈkla-bərd;ˈkla(p)-ˌbȯrd\ n {part trans. of D klaphout stave wood} (ca. 1520) 1 archaic : a size of board for making staves and wainscoting 2 : a narrow board usu. thicker at one end edge than the other, used for siding 3 \ˈklap-ˌbȯrd\ : a pair of hinged boards one of which has a slate with data identifying a piece of film and which are banged together in front of a motion picture camera at the start of a take to facilitate editing — called also clapper board

So we have two nouns that came into the English language at approximately the same time, with the same pb combination at the junction of the two syllables and with the same -board ending. And yet, whereas the Eleventh Collegiate lists the only pronunciation of cupboard as \ˈkə-bərd\ , it lists three pronunciations for clapboard: \ˈkla-bərd\ , \ˈkla(p)-ˌbȯrd\ , and (for the much later meaning of the word) \ˈklap-ˌbȯrd\ .

A discussion headed "Clapboard & Cupboard" on a page of Merriam-Webster's site emphasizes the etymological differences between the two words, but makes no attempt to tie those differences to the variable pronunciations of clapboard versus the unitary pronunciation of cupboard:

Cupboard literally is a "cup board": that is, a board or table on which cups can be stored—at least at its origins in the Middle Ages. The "closet" meaning dates to the mid-1500s, and the "p" and "b" of the spelling have long since merged in pronunciation. As an exercise, try saying the literal "cup board" ten times fast, and you’ll experience firsthand how language evolves.

Clapboard has a different story: it came to English as a partial translation of the Dutch word klaphout, meaning "stave wood"; it probably derives from the Dutch verb clappen, meaning "to clap" or "to hit," from the way carpenters nailed the siding to houses. The literal pronunciation is sometimes used for the wood siding but always used when clapboard refers to the clapping slate used in filmmaking.

I suspect that the pronunciation of cupboard progressed through each of the three stages that clapboard exhibits, in this order: \ˈkəp-ˌbȯrd\ , \ˈkə(p)-ˌbȯrd\ , \ˈkə-bərd\ . But cupboard is a more common word than clapboard, and that may help explain why the slurring loss of the p sound occurred more completely and perhaps at an earlier date with cupboard than with clapboard. The completeness of the loss of the p sound may also owe something to the fact that almost every native English speaker first encounters cupboard in spoken English (such as in "Old Mother Hubbard") rather than in writing, reducing the possibility of a spelling-influenced pronunciation.

As for the mechanics of the transition from clearly sounded p to dropped p in the case of cupboard (and one pronunciation of clapboard), I defer to the answer posted by tchrist, who has far deeper understanding than I do of this aspect of language.