To be thirsty, is to over actively: investigate, long for, covet, in some occasions intensely staring, or bold displays of desperation.
Jim: "Could you believe the gull of that one chick?"
Matt:" I know! She went above, and beyond to get next to you! She didnt care what anyone would think of he efforts."
Jim: "Yeah... she was so thirsty."
(verb) When someone lacks interaction with the opposite/attracted sex. They would call anyone hot when usually they wouldn't think so, it's just the thirst that's got a hold of them.
"There is an actual boy at my house. He's so hot!"
"No he's not. You're just thirsty"