Have no doings with the Erythynus. This is the thirty-third Symbol of the Protreptics of Iamblichus. The Erythynus is a fish called by Pliny (ix. 77) erythrinus, a red fish with a white belly. Pythagoras used this fish as a symbol of a braggadocio, which has a lily liver. Have no doings with those who are tongue-doughty, but have white stomachs (where stomach means true courage).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894