ὑγιέστερος κροτῶνος → fit as a fiddle, as fit as a fiddle
There was a proverb ὑγιέϲτεροϲ κροτῶνοϲ ('a tick'), used by Menander in his Λοκροί (frag. 263). Zenobios vi. 27 explains that this animal is all smooth, without scratch or other harm. But Strabo vi. 1. 12 believes the proverb to be ὑγιέϲτεροϲ Κρότωνοϲ, the healthiness of Kroton being testified to by the multitude of athletes it produced. Zereteli saw a note in the right-hand margin which he dubiously read as τί λ[έγειϲ;] ὑγι[αι]ν[ει]. Sudhaus supposed that the sense was: 'Is he well?—Fit as a fiddle.—What's that?—I say he is as fit as a fiddle.'
Quoted by Diogenes Laertius 6.83 (215 = K-A 193) - Menander - Oxford Scholarly Editions