1520s, from French insupportable (14c.) or directly from Late Latin insupportabilis, from in- "not" (see in- (1)) + *supportabilis, from Latin supportare "to carry" (see support (v.)). Related: Insupportably.
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.
late 14c., "to aid," also "to hold up, prop up, put up with, tolerate," from Old French suporter "to bear, endure, sustain, support" (14c.), from Latin supportare "convey, carry, bring up, bring forward," from assimilated form of sub "up from under" (see sub-) + portare "to carry," from PIE root *per- (2) "to lead, pass over." Related: Supported; supporting.
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Harper, D. (n.d.). Etymology of insupportable. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/insupportable