Green’s Dictionary of Slang

crack adv.

[crack adj. (1)]

1. in a first-rate manner.

[US]Eve. Star (N.Y.) 28 Apr. 2/2: For, in beauty and perfection of model, our ‘New York crack built ships,’ as they are familiarly called on the ocean, and in all parts of the world, so far excel the efforts of other people, that they have become . . . models of naval architecture.

2. incontrovertibly, absolutely.

[UK]J. Bowes Jackaroos 240: You’re crack sure, Aleck?