consonantal digraph now in English usually representing the sound of -f-, originally it was the combination used by Romans to represent Greek letter phi (cognate with Sanskrit -bh-, Germanic -b-), which at first was an aspirated "p," later probably the same sound as German -pf-. But by 2c. B.C.E. had become a simple sound made by blowing through the lips (bilabial spirant).
Roman "f," like modern English "f," was dentilabial; by c. 400, however, the sounds had become identical and in some Romanic languages (Italian, Spanish), -ph- regularly was replaced by -f-. This tendency took hold in Old French and Middle English, but with the revival of classical learning the older words subsequently were altered back to -ph- (except fancy and fantastic), and due to overcorrection in this some non-Greek words in -f- began to appear confusedly in -ph-, though these forms generally have not survived (nephew is an exception). The modern slang fad for replacing f- with ph- (as in phat) seems to date to the 1960s and phone phreak (see phreak), where it might have been suggested by the spelling of (tele)phone.
1550s, "the letter F;" 1690s as the name of a former letter in the Greek alphabet, corresponding to -F- (apparently originally pronounced with the force of English consonantal -w-), from Latin digamma "F," from Greek digamma, literally "double gamma" (because it resembles two gammas, one atop the other). The sixth letter of the original Greek alphabet, it corresponded to Semitic waw.
by 1943, the name of the letter -f-, used as a (presumably) euphemistic abbreviation of fuck (q.v.) down to its first letter. Related: Effing.
1873, "fondling, indulgence," verbal noun from pet (v.). Meaning "amorous caressing, foreplay" is from 1920 (in F. Scott Fitzgerald).