According to Google at least, the word "cupboard" originated in late Middle English as denoting a board that held cups. Since then, the word has evolved to mean a kind of cabinet.
My question is, given its origin and spelling, why do we pronounce "cupboard" with a silent "p"? Has the pronunciation simply evolved because "cup-board" is too awkward to say, or is there a deeper pronunciation rule that I'm not aware of?
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:
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.
Technically, both are pronounced. But since the sound of letter 'b' dominates the sound of letter 'p', we feel that p is silent. I am not aware of any such rule where the letter 'b' should be silent.
