The story
Comb is named for its teeth. Old English camb goes back to an ancient root meaning "tooth" — the same root behind Greek gomphíos, "molar". German Kamm and Dutch kam are the same word. A comb is, quite literally, "the toothed thing".
Two sounds in it have changed. The B was pronounced: the word ended in "-mb". In later Middle English, final -mb was reduced to plain M — the change that also silenced lamb and climb. And the vowel moved. Old English lengthened vowels before -mb, so the short "a" of camb became a long vowel that developed into the "oh" of modern comb.
The spelling b survives from before the change. Notice that comb and tomb and womb are all spelled -omb but each has a different vowel today — the spelling is older than all three pronunciations.
