late 14c., from Old French impossible (14c.), from Latin impossibilis "not possible," from assimilated form of in- "not, opposite of" (see in- (1)) + possibilis (see possible). Weakened sense of "unable to be accomplished or tolerated" is from mid-19c. Related: Impossibly.
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.
<a href="https://www.etymonline.com/word/impossibilism">Etymology of impossibilism by etymonline</a>
Harper, D. (n.d.). Etymology of impossibilism. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/impossibilism