Etymology of “hairdo” is pretty unsatisfying — from “doing” one’s hair. And yet there’s something curious about its formation: one’s hair is “done” — past perfect tense? – yet the finished product is a hair “do” — present tense.
It isn’t a hair done or done-hair or do-hair. (And “Doo-wop” is unrelated.)
Just looking at the word without knowing what it means, one might guess that it refers to what hairs do, what hairs are made to do — how hairs are made to be in a certain way. Maybe hair-be would be a more accurate term.
Imagined conversation:
— I really love your hairbe.
— Thanks, I just had it done.
Well, maybe “hairdo” makes some sense after all. I think “hair style” is still more sensible. And it’s unusual that “hairdos” are only really for ladies.