It is sometimes used to refer to someone who has unusually high energy, speed, or productivity - to accuse them of using a stimulant.

> To be crack -- as in "a crack shot" or "a crack team of commandos"

To explore the quirks of English grammar a bit further... While "the general expected this to be a crack team of commandos" flows fine, I've never seen "the general expected this team to be crack".

Perhaps it's similar to the rules for the adjective "top"? One can have a "top team of athletes", but "this team of athletes is top" doesn't work, it needs to be "topmost" or something.

IANAGrammarian, but it's almost as if "crack" isn't being used as adjective that modifies "team", but instead they've merged to become a single noun of "crack-team"... kind of like how "first-responder" is a category of emergency-related jobs rather than literally any person who happens to be the first to arrive.

I believe it is derived from "crackerjack".

Crack cocaine has a wide range of effects. Some people just get energetic, others become disorganized to the point of extreme dysfunction.

To be "on crack" is to display any of these qualities; sometimes good, sometimes bad.

I usually see it used to describe a positive trait in the form of high energy / confidence.