A women of the looser variety who's inclined to keep company with many different men in the armed forces. Especially Marines.
Person 1: "There's Meg. Wasn't she with a Navy guy last week? She must have a thing for guys in uniform"
Person 2: "Yeah she's a Joe Jumper. She's dating a Marine now."
Person 1: "Yeah those Joe Jumpers sure do love seamen."