Quid hoc ad Iphicli boves? What has that to do with the subject in hand? So in L'Avocat the judge had to pull up the shepherd every minute with the question, “Mais, mon ami, revenon à nos moutons.” Iphiclos or Iphicles was the possessor of large herds of oxen, and Neleus (2 syl.) promised to give his daughter in marriage to Bias if he would bring him the oxen of Iphicles, which were guarded by a very fierce dog. Melampos contrived to obtain the oxen for his brother, but being caught in the act, he was cast into prison. Melampos afterwards told Astyocha, wife of Iphicles, how to become the mother of children, whereupon Iphicles gave him the coveted herd, and his brother married the daughter of Neleus. The secret told by Melampos to Astyocha was “to steep the rust of iron in wine for ten days, and drink it.” This she did, and became the mother of eight sons.
(Odyssey, xi.; Iliad, xiii. 23; Apollodoros, i. 9; Pausanias, iv. 36.)
When Tressilian wanted Dominie Holiday to tell him of a smith who could shoe his horse, the pedagogue kept starting from the point, and Tressilian says to him:
“Permit me to ask, in your own learned phrase, Quid hoc ad Iphycli boves, what has that to do with my poor nag?” —
Another similar phrase is “Quid ad Mercurium?”
Another is “Io Hecuba?” What has that to do with Hecuba?
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894