"fasten, join, attach," 1530s, from Medieval Latin affixare, frequentative of Latin affigere (past participle affixus) "fasten to, attach," from ad "to" (see ad-) + figere "to fasten" (from PIE root *dheigw- "to stick, fix").
According to OED first used by Scottish writers and thus perhaps the immediate source was French affixer, a temporarily re-Latinized spelling of Old French afichier (Modern French afficher), from a Medieval Latin variant of the Latin verb. The older form in English was affitch (Middle English afficchen, late 14c.), from Old French afichier. Related: Affixed; affixt; affixing.
Proto-Indo-European root meaning "to stick, fix."
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dehi- "wall;" Old Persian dida "wall, stronghold, fortress," Persian diz; Latin figere "to fix, fasten, drive, thrust in; pierce through, transfix;" Lithuanian dygstu, dygti "germinate;" Old Irish dingid "presses, thrusts down;" Old English dic "trench, ditch," Dutch dijk "dam."