To fumble is to not
date someone you could have because of various reasons. You
always come to regret it. You either didn't know they liked you when everyone else did, said something weird in
front of them etc..
And it sucks ass.
Person 1 "Man!! Everyone knew she liked you!! All you had to do was
ask, how could you fumble this
hard?"
Person 2 "I know, I fumbled so
hard."