demonstrable (adj.)

active word-forming element in English and in many verbs inherited from French and Latin, from Latin de "down, down from, from, off; concerning" (see de), also used as a prefix in Latin, usually meaning "down, off, away, from among, down from," but also "down to the bottom, totally" hence "completely" (intensive or completive), which is its sense in many English words.

As a Latin prefix it also had the function of undoing or reversing a verb's action, and hence it came to be used as a pure privative — "not, do the opposite of, undo" — which is its primary function as a living prefix in English, as in defrost (1895), defuse (1943), de-escalate (1964), etc. In some cases, a reduced form of dis-.

early 14c., monstre, "malformed animal or human, creature afflicted with a birth defect," from Old French monstre, mostre "monster, monstrosity" (12c.), and directly from Latin monstrum "divine omen (especially one indicating misfortune), portent, sign; abnormal shape; monster, monstrosity," figuratively "repulsive character, object of dread, awful deed, abomination," a derivative of monere "to remind, bring to (one's) recollection, tell (of); admonish, advise, warn, instruct, teach," from PIE *moneie- "to make think of, remind," suffixed (causative) form of root *men- (1) "to think."

Abnormal or prodigious animals were regarded as signs or omens of impending evil. Extended by late 14c. to fabulous animals composed of parts of creatures (centaur, griffin, etc.). Meaning "animal of vast size" is from 1520s; sense of "person of inhuman cruelty or wickedness, person regarded with horror because of moral deformity" is from 1550s. As an adjective, "of extraordinary size," from 1837. In Old English, the monster Grendel was an aglæca, a word related to aglæc "calamity, terror, distress, oppression." Monster movie "movie featuring a monster as a leading element," is by 1958 (monster film is from 1941).

1560s, from in- (1) "not, opposite of" + demonstrable.