impregnable (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.

of a fortress, etc., "capable of being taken or won by force," 1530s, an alteration of Middle English preignable, earlier prenable (early 15c.), pernable (late 14c.), from Old French prenable, pregnauble "assailable, vulnerable," from stem of prendre "to take, grasp, seize," from Latin prehendere "to take hold of, to seize" (from prae- "before," see pre-, + -hendere, from PIE root *ghend- "to seize, take"). The form was confused in French and English by the influence of unrelated words from French preignaunt and English pregnant.

1755, from impregnable + -ity.