Green’s Dictionary of Slang

fret n.

In phrases

on the fret (adj.)

perturbed, worried.

1826

Pierce Egan’s Life in London 21 May 549/1: The Bonnifaces on the road-side appeared all upon the fret [...] anxiously upon the look-out to pick up a straggler, to buy a thimbleful of summut, or to stop and get a bit of feed and a whet for the prad.