1. (Adj) A person or group whose sexuality is in question, or (specifically) whose sexual preference seems to fluctuate on a weekly basis.
Kelly: "Andrew, WTF? Why is Dave ignoring me and making out with a dude?!"
Andrew: "It's Sunday."
Kelly: "What is that supposed to mean? Last night we hooked up and he made all these homophobic remarks... and now all of a sudden he's gay?"
Andrew: "Yeah, didn't you know, Dave's biweekly."