To be "in lust" with someone is to crave someone sexually or physically without necessarily liking their personality or who they are. Basically a physical version of "in love".
Johnny: So dude, how did you ever like Christy? She was a total bitch!
Nelson: I didn't care about that. I was just in lust with her.
Alex: Hey Katie, I don't really like you, but I'm in lust with you.
Katie: Same here! Let's just hook up.