a girl who is pretty(face), not sexy(body), not both, causing men to ask eachother "Would you fuck her?" rather than assuming the answer is yes like they would if she were both. adding in the "ma" makes it sound more like a word than "wouldyafucker".
Bill: "That girl is pretty. Would you fuck her?"
Bob: "Nah, her tits are way too tiny."
Bill: "Yeah i guess she is a wouldyamafucker."