Clipping of tachometer
tach (plural tachs)
Backslang for hat (with /tʃ/ substituted for the /h/).
tach (plural taches)
- (obsolete, back slang) A hat.
c. 1864, Stevens, Alfred Peck, “The Chickaleary Cove”, in Farmer, John Stephen, editor, Musa Pedestris, published 1896, page 161:
- I have a rorty gal, also a knowing pal, / And merrily together we jog on, / I doesn't care a flatch, as long as I've a tach, / Some pannum for my chest, and a tog on.