Green’s Dictionary of Slang

1. an exhortation to make an effort, to overcome one’s fears, to get on energetically.

1977		Eble Campus Sl. Apr. 2: go for it – to try very hard to win [...] A: You think I can hit a home run? B: Go for it!
1983		A-Team Storybook 25: Let’s go for it, boys!
1991		Desperate Dan Special No. 7 13: Let’s go for it!
2003		McGill Reed Dict. of N.Z. Sl.

2. a general exhortation to those present, urging them to act crazily, the intention being thus to have fun.

1978		Eble Campus Sl. Apr. 2: go for it – take a chance; it might be fun; you only live once.
1999		K. Sampson Powder 159: Go for it. Ride that bronco for all it’s worth, yeah?
2000		N. Barlay Crumple Zone 105: Go for it girl.