Mansuetude \MAN-swi-tood\ , noun;
1. Mildness; gentleness
This word dates to the late 14th century and comes from Latin mansuetudo ("tameness"), which comes from mansuetus (past participle of mansuescere, "to tame"). Mansuescere literally means "to accustom to the hand" from manus + suescere ("hand" + "to accustom, habituate") and comes from Proto-Indo-European *swdh-sko- from the base *s(w)e-.
Oddly enough, *s(w)e- is also the bases for the Modern English word idiom. Here's the progression:
*s(w)e- > *swed-yo- > idios > idioumai > idioma > idioma > idiome > idiom
PIE > PIE > Greek "personal, private" > Greek "I make my own" > Greek "peculiarity, peculiar phraseology" > Late Latin "a peculiarity in language" > Modern French "form of speech peculiar to a people or place" > English
Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Wednesday, January 12
Etymologies come from the Oxford English Dictionary and/or Etymonline.com