insoluble 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.

late 14c., "unconstipated;" early 15c., "capable of being dissolved," from Old French soluble "expungable, eradicable" (13c.), from Late Latin solubilis "that may be loosened or dissolved," from stem of Latin solvere "to loosen, dissolve," from PIE *se-lu-, a compound of reflexive pronoun *s(w)e- (for which see idiom) + root *leu- "to loosen, divide, cut apart." The meaning "capable of being solved or explained" is attested from 1705 (Berkeley). Substances are soluble, but not solvable; problems can be either.

1754, "incapability of dissolving in a liquid," 1791, from Late Latin insolubilitas, from Latin insolubilis "that cannot be loosened" (see insoluble). Earlier in a sense "that cannot be dissolved" (of marriage), 1610s. The meaning "that cannot be solved" (1722) probably is a separate formation from insoluble.