Usually occurs from a man or boy between the months of May all the way to September. A testosterone boner is when a man notices a very attractive girl wearing minimal clothing and all the "alerts" of a boner go off minus the inconvenient and obvious side effect.
"Dude, G Thom, did you see that fine girl in the short shorts?"
"Hell yeah I did, damn girl gave me a testosterone boner."
"but you're not showing?"
"exactly, doesn't mean I'm not poppin' on the inside."