What does it mean when a guy grabs a girls thigh?

Too many factors to give a definitive answer on this one.

• It really depends on the type of grab. If it's firm and playful, it may be friendly.
• If there's rubbing involved them they've got something else in mind.
• If it hurts then they may not like you, or maybe they do, and they're just into that kind of thing.

Location of the grab has to be taken into consideration as well. If it's in an erogenous zone, then I hope you've got an understanding with the grabber.

Also depends on how close your relationship is to the grabber. Are you friends, or more? If they're your Uncle - then tell your parents. If it's your Dad and it's unwelcome and uncomfortable - tell your Mum or the police.