some guy who just tags along or goes with what ever you say just to get in your pants. He is dependent on you, but once it gets boring he'll hop over to someone else's bed or pants (whatever you prefer). A dick-along is also what an envious boy or girl says when they are jealous of somebody's so called loyal boyfriend
Max: Wow John is whipped.
Fred: Nah, he's just Amy's dick-along.
Ally: Did you ever meet Jessie's new boyfriend
Jane: Oh' you mean Kevin?
Ally: Yea isn't he sweet?
Jane: *scoffs* no he's just a dick-along (she's thinking he really belongs with her).