c. 1300, "attractiveness, beauty, charm" (archaic), from Old French favor "a favor; approval, praise; applause; partiality" (13c., Modern French faveur), from Latin favorem (nominative favor) "good will, inclination, partiality, support," coined by Cicero from stem of favere "to show kindness to," from PIE *ghow-e- "to honor, revere, worship" (cognate: Old Norse ga "to heed").
Meaning "good will, kind regard" is from mid-14c. in English; sense of "act of kindness, a kindness done" is from late 14c. Meaning "bias, partiality" is from late 14c. Meaning "thing given as a mark of favor" is from late 15c. Phrase in favor of recorded from 1560s.