epexegesis n.

before vowels reduced to ep-, before aspirated vowels eph-, word-forming element meaning "on, upon, above," also "in addition to; toward, among," from Greek epi "upon, at, close upon (in space or time), on the occasion of, in addition," also "after," from PIE *epi, *opi "near, at, against" (source also of Sanskrit api "also, besides;" Avestan aipi "also, to, toward;" Armenian ev "also, and;" Latin ob "toward, against, in the way of;" Oscan op, Greek opi- "behind;" Hittite appizzis "younger;" Lithuanian ap- "about, near;" Old Church Slavonic ob "on"). A productive prefix in Greek; also used in modern scientific compounds (such as epicenter).

1610s, "explanatory note," from Greek exegesis "explanation, interpretation," from exegeisthai "explain, interpret," from ex "out" (see ex-) + hegeisthai "to lead, guide," from PIE root *sag- "to track down, seek out" (see seek (v.)). Meaning "exposition (of Scripture)" is from 1823. Related: Exegetic; exegetical; exegetically.