When someone is dating someone that is either too ugly, old, young, or just plain wrong for them. It can also be used when you're dating someone in a much lower socio-economic class or someone of a different race or level of intelligence.
Fred: Dude, so who're you into these days?
Tim: This freshman in high school. She's really cute.
Fred: Gross... you're a sophomore in college. Way to start dippin' in the fondue.
Katie: What do you think about me dating Joe?
Jen: Are you serious? He's so fugly. You'd be hardcore dippin' in the fondue.
The playmates who date Hugh Hefner
The late Anna Nicole Smith with her rich, old, crusty husband