woman is a contraction of its original form, wife-man, so wo, in a sense, stands for wife.

EDIT: I can see that, as often happens here, people are substituting fiction for fact. It is indeed wife-man in its original form, or the equivalent of that, and here is what the authoritative OED says about the etymology:

"OE. wífmon(n, -man(n masc., later fem., pl. wífmen(n, f. wíf woman, WIFE n. + mon(n, man(n human being, MAN n.1 A formation peculiar to English, and not extant in the earliest period of OE., the ancient word being WIFE."

There you have it. "Woman" started out as "Wife-man"