When a guy has sex with two or more girls at the same time.
Used to be that just getting laid was getting lucky; now, if the girl you've got your dick in doesn't have her tongue in another girl's pussy, you're barely gettin' laid. Man I love the new millennium.
getting lucky, similar to getting some refers to having sex. Usually uses by females to express their willingness to share the bed with someone, or as a comment by males to say that they hope to get a girl into bed later on, see examples.
Woman: "I think you're getting lucky tonight.
Or: Man: "I think I'm getting lucky tonight with Judy".