LSJ
τό, name of the letter φ, Callias ap. Ath. 10.453d, Hellad. ap. Phot. Bibl. 530B. ; φεῖ shd. be restored for φῖ in Pl. Cra. 427a, etc. ; a pun on φφ = Φιδίς (leg. Φειδίς) in AP 7.429 (Alc.).
Liddell-Scott-Jones, Greek-English Lexicon (9th ed., 1940)