To be undecided or indecisive, to switch back and forth between decisions, opinions, options, or directions.
You're such a wembler, Wembley!
Wembley is one of the characters in Fraggles. He's known to be very indecisive. Wemble is a Fraggle verb and means not being able to decide.
Girl: Would you like pizza or the big salad?
Guy: I don't know. Pizza is tasty, but then again, salad is healthy. I'm hungry, so pizza would be nice. But the salad looks great. Maybe I'll take the salad then. Uh, but I would really like the pizza...
Girl: Don't wemble like a moron. Come on, what do you want?