Woman was originally spelled wifmann, from wif meaning "female" and mann meaning "person". The f eventually became an m, making wimmann.

Wif was pronounced "weef". The "ee" vowel is pronounced close to the front of the mouth. Over the centuries, the pronunciation of wimman changed, the vowel moving farther back in the mouth from an "ee" to an "ih" (as in "little"), then farther back to an "ooh" (as in "book").

The reason it's not pronounced with an "aww" (as in "yawn") or an "ahh" (as in "thought") is that these vowels are made even farther back in the mouth, close to the throat. They're also made with the mouth farther open than "ee", "ih", or "ooh". The vowel may have drifted from "ee" over the centuries, but it did not drift far enough to reach "ahh".