1909, from German PH, introduced by S.P.L. Sörensen, from P, for German Potenz "potency, power" + H, symbol for the hydrogen ion that determines acidity or alkalinity.
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.