Green’s Dictionary of Slang

1. the fashion.

1840

T. Haliburton Clockmaker III 203: I have had liberal offers from the sect here, for whatever is the go to Europe will soon be the chalk here.

2. the absolute truth.

1844

‘Jonathan Slick’ High Life in N.Y. II 202: I tell you what, it was the clear chalk, the ginuine thing.