tach /tæk/noun 2/tæt͡ʃ/ noun 1forms 2phrases 1 noun2 •(informal) Tachycardia. •(informal) Clipping of tachometer. tach /tæt͡ʃ/ noun1 •(no longer used) A hat.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. — 1864 formstaches · tachs Phrases with tach tach upTo rev (up) a car's engine; to accelerate.