Etymology
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inhuman (adj.)
mid-15c., "cruel," from Latin inhumanus "inhuman, savage, cruel, rude, barbarous, uncultured," from in- "not, opposite of" (see in- (1)) + humanus "human" (see human (adj.)). Spelled inhumane till 18c. (see humane).
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inhumane (adj.)
originally a variant spelling and pronunciation of inhuman "cruel, hard-hearted;" it appears to have died out 17c. but returned c. 1822, probably a reformation as a negative of humane (q.v.), with its accent.
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inhumanity (n.)

"barbarous cruelty," late 15c., from French inhumanité (14c.) or directly from Latin inhumanitatem (nominative inhumanitas) "inhuman conduct, savageness; incivility, rudeness," noun of quality from inhumanus "inhuman, savage, cruel" (see inhuman).

And Man, whose heav'n-erected face
The smiles of love adorn,—
Man's inhumanity to man
Makes countless thousands mourn!
[Robert Burns, "Man was Made to Mourn," 1784]
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humane (adj.)
mid-15c., a parallel variant of human (adj.), with a form and stress that perhaps suggest a stronger association with Latin humanus than with Old French humain. Human and humane were used interchangeably in the senses "pertaining to a human being" and "having qualities befitting human beings" (c. 1500). The latter at first meant "courteous, friendly, civil, obliging," then "marked by tenderness, compassion, and a disposition to kindly treat others" (c. 1600). By early 18c. the words had differentiated in spelling and accent and humane took the "kind" sense.

Compare germane, urbane. Meaning "inflicting less pain than something else" is from 1904. Inhuman is its natural opposite. The Royal Humane Society (founded 1774) was originally to rescue drowning persons; such societies had turned to animal care by late 19c.
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barbarity (n.)
1560s, "want of civilization," from Latin barbarus (see barbarian (n.)) + -ity. Meaning "savage cruelty, inhuman conduct" is recorded from 1680s.
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brutality (n.)
1540s, "quality of resembling a brute;" 1630s, "savage cruelty, inhuman behavior, insensibility to pity or shame," from brutal + -ity. Literal sense "condition or state of a brute" is from 1711.
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brutalize (v.)
"make coarse, gross, or inhuman, lower to the level of a brute," 1740, from brutal + -ize. Related: Brutalized; brutalizing. An earlier verb was brutify (1660s), from French brutifier. Related: Brutification.
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brutal (adj.)
mid-15c., "bestial, pertaining to or resembling an animal" (as opposed to a man), from Old French brutal, from Latin brutus (see brute (adj.)). Of persons, "unintelligent, unreasoning" (1510s); "fierce, savage, cruel, inhuman, unfeeling" (1640s). Related: Brutally.
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unmanly (adj.)
late 15c., "degrading to a human,"from un- (1) "not" + manly (adj.). Similar formation in Middle Dutch onmamlijc, German unmännlich. Meaning "not having the qualities or attributes of a man" (as opposed to a woman or child) is from 1540s. Old English had unmennisclic "inhuman" (adj.); unmann (n.) "monster; wicked man."
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monster (n.)
Origin and meaning of monster

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).

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