lit. 'accidental' (from upapāta, accident; not from upapatti, as PTS Dict. has); 'spontaneously born', i.e. born without the instrumentality of parents.
This applies to all heavenly and infernal beings. "After the disappearing of the 5 lower fetters (samyojana, q.v.), he (the Anāgāmi) appears in a spiritual world (opapātika) ...."