incombustible (adj.)

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

"capable of burning," 1520s, from French combustible, or directly from Late Latin combustibilis, from Latin combustus, past participle of combuere "to burn up, consume" (see combustion). Figurative sense "easily excited" is from 1640s. As a noun, "a substance that will burn," from 1680s. Related: Combustibility (late 15c.).