ab- Look up ab- at Dictionary.com
word-forming element meaning "away, from, from off, down," denoting disjunction, separation, departure; from Latin ab-, 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 "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-; sometimes abs- before -c- or -t-.