A female who has highly defined biceps likened to that of males. Said female most likely also has a very defined six-pack, more defined than your boyfriend's in fact.
It is often the sign of a lesbian, any woman who practices Wing Chun, or Ms. Universe.
Mike: Man, check out that girl's guyceps!
David: I now know why they call it Manchester.
"Hey, did you see how hot that girl was?"
"Dude, she's tots a lesbo."
"Just cause her name is Leslie---"
"Duuuuude, guyceps. DUH."
a muscle that grows exclusively on the male human, aka "the penis"
Dave: Man your forearms are huge!
Matt: Yeah, been working out the Guy-cep lately
When a girl has jacked arms, similar to a man's.
That girl has wicked guyceps!