"a hollow place, empty space in the body," 1540s, from French cavité (13c.), from Late Latin cavitatem (nominative cavitas) "hollowness," from Latin cavus "hollow" (from PIE root *keue- "to swell," also "vault, hole").
1610s, "dull in the mind, flat, insipid, wanting in interest," from Latin ieiunus "empty, dry, barren," literally "fasting, hungry," a word of obscure origin. De Vaan finds it to be from a PIE root meaning "to worship, reverence," hence "to sacrifice" (with cognates including Sanskrit yajati "to honor, worship, sacrifice," Avestan yaza- "to worship," Greek agios, agnos "holy;" see hagio-), and writes that the Latin word and its relatives "would be based on the habit to perform the first sacrifice of the day on an empty stomach." Related: jejunal; jejunally.
Earliest sense in English is medical. Military use is by 1710. Meaning "remove inhabitants to safer ground" is from 1934. Intransitive sense is from 1630s; of civilian persons by 1900. Replaced Middle English evacuen "draw off or expel (humors) from the body" (c. 1400). Related: Evacuated; evacuating.
Old English idel "empty, void; vain; worthless, useless," from Proto-West Germanic *idla- (source also of Old Saxon idal, Old Frisian idel "empty, worthless," Old Dutch idil, Old High German ital, German eitel "vain, useless, mere, pure"), a word of unknown origin.
Subsequent developments are peculiar to English: sense "not employed, not doing work" was in late Old English in reference to persons; from 1520s of things; from 1805 of machinery. Meaning "lazy, slothful" is from c. 1300. In Elizabethan English it also could mean "foolish, delirious, wandering in the mind." Idle threats preserves original sense.