abnormal adj.

"not according to rule, abnormal," 1812, from French anormal, from Medieval Latin anormalus, a corruption of anomalus (from Greek anomalos "uneven, irregular;" see anomaly), as if from Latin ab "away" + norma "rule, pattern." In 20c., it was used as "an antonym of 'normal' when the association of 'abnormal', i.e. 'unhealthy', 'unnatural', would be inappropriate" [OED].

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-.