"deny (something) to oneself," 1650s, from Latin abnegatus, past participle of abnegare "to refuse, deny," from ab "off, away from" (see ab-) + negare "to deny," from PIE root *ne- "not." Related: Abnegated; abnegating.
word-forming element meaning "away, from, from off, down," denoting disjunction, separation, departure; from Latin ab (prep.) "off, away from" in reference to space or distance, also of time, from PIE root *apo- "off, away" (also the source of Greek apo "off, away from, from," Sanskrit apa "away from," Gothic af, English of, off; see apo-).
The Latin word also denoted "agency by; source, origin; relation to, in consequence of." Since classical times usually reduced to a- before -m-, -p-, or -v-; typically abs- before -c-, -q-, or -t-.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit a-, an- "not;" Avestan na "not;" Greek a-, an-, ne- "not;" Latin in- "not," ne "that not;" Old Church Slavonic and Lithuanian ne "not;" Old Irish an-, ni, Cornish ny "not;" Gothic and Old English un- "not."
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Harper, D. (n.d.). Etymology of abnegate. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/abnegate