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."