Definitions by Mr. Men

Vitamin G 

The only 'vitamin' on earth that will actually give you gains, hence the G.

i.e. steroids.

Bob: Jesus Christ, I've been getting no gainz lately. I'm thinking of hittin up some supplements or something.
Jim: Dude...there's no way to look like those guys from taking supps, they're all on vitamin G.

Vitamin G by Mr. Men December 6, 2016