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late 15c., from French inhumanité (14c.) or directly from Latin inhumanitatem (nominative inhumanitas) "inhuman conduct, savageness," noun of quality from inhumanus (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]