Latin, literally "in (his/her/their) absence" (see absence). By 1831 in English, earlier in legal Latin.
Entries linking to in absentia
absence n.
"state of not being present," late 14c., from Old French absence "absence" (14c.), from Latin absentia, abstract noun from absentem (nominative absens), present participle of abesse "be away from, be absent," from ab "off, away from" (see ab-) + esse "to be" (from PIE root *es- "to be"). Absence makes the heart grow fonder is a line from the song "Isle of Beauty" by English poet and composer Thomas Haynes Bayly (1797-1839).