Confidence boost is when a really shy person gets really confident and doesn't care about any consequences of any actions, it always lasts for a day or two. Later the person regrets it.
Jill - Didn't Joe use to be the quietest kid in school? Why is he suddenly so confident now?
Dan - I think he got a confidence boost
Jill - Oh....This better end well
by The biggest Grammar Nazi October 19, 2015
First girlfriend confidence boost or FGCB is a condition that occurs when someone who is socially awkward and has no history with women finally gets a girlfriend. They person will begin to think they are cool, even if there girlfriend is unattractive and they will think they are "the man" because their penis gets touched for the first time, failing to realize that everyone's has by now. They may try to act like a tough guy because they are taller than their girlfriend when in reality she is most likely just really short. The only cure for FGCB is a breakup which will destroy this newly found ego.
Jake kept talking about how much action he's getting, then tried to fight me, I think he's got a case of the First Girlfriend Confidence Boost.
by The Hershey Factory May 14, 2012