member of the supreme college of priests in ancient Rome, 1570s, from Latin pontifex "high priest, chief of the priests," probably from pont-, stem of pons "bridge" (see pons) + -fex "maker," from facere "to do, make" (from PIE root *dhe- "to set, put").

If so, the word originally meant "bridge-maker," or "path-maker." It was felt as such; the sense of "bridge-builder" was in the Medieval Latin word, and Milton uses pontifical (adj.) in this sense. Sense was extended in Church Latin to "a bishop," in Medieval Latin to "the Pope." In Old English, pontifex is glossed in the Durham Ritual (Old Northumbrian dialect) as brycgwyrcende "bridge-maker." 

Weekley points out that, "bridge-building has always been regarded as a pious work of divine inspiration." Century Dictionary speculates it had its origins as "having charge of the making or maintenance of a bridge — it is said of the Sublician bridge built over the Tiber by Ancus Marcius." Or the term may be metaphoric of bridging the earthly world and the realm of the gods. Other suggestions trace it to Oscan-Umbrian puntis "propitiary offering," or to a lost Etruscan word; in either case it would have been altered by folk etymology to resemble the Latin for "bridge-maker."