He's probably just copying stuff he's heard from the rest of the crew. You can repeat a figure of speech knowing what it means without understanding why.

In film time, you can learn anything.
Whatever you are, be a good one.

Seriously? How did I miss that?
Oh well. Still plenty of time for them to hammer it home.
Quidquid Latinae dictum sit, altum viditur.

3 years? It would have been a couple months. James Gunn confirmed that GOTG 2 takes place two months after GOTG 1.
Urich: Guess I have more faith in humanity. Fisk: So did Christ, if I recall.

Well, he'll have hung out with Rocket and Quill for about three years.
Quidquid Latinae dictum sit, altum viditur.

He learned "screw you" it seems.
Whatever you are, be a good one.

The scene when he kills Korath the Pursuer shows him understanding the concept of a metaphor, but that he hasn't got the usage quite down yet.
Quidquid Latinae dictum sit, altum viditur.

Maybe he's become more acclimated to the idea after hanging with the likes of Star-Lord and Rocket for so long.
Whatever you are, be a good one.

A real missed comedic opportunity though.
And any questioning of the metaphor from Drax would have totally killed the entire scene.
GotG knows when to be funny, but more importantly it knows when
not
to be funny.
Quidquid Latinae dictum sit, altum viditur.

Drax was already understanding metaphors by the end of the first movie. Notice how when Quill gives his big speech about how they need to "give a sh!t" Drax completely fails to sh!t in his own hand and hand it to Quill? He apparently understood that metaphor. A real missed comedic opportunity though.

Quill must have taught him how to understand and use metaphors now he's using them so much.

Well, he sort of used one at the end of the first movie, so he seemed to be catching on. Being around these people a couple more months would certainly rub off on him.