Why is the ninth month called September?
c.1600, past participle adjective from oblige. To be obliged "be bound by ties of gratitude" is from 1540s.
c.1300, "to bind by oath," from Old French obligier "engage one's faith, commit (oneself), pledge" (13c.), from Latin obligare "to bind, bind up, bandage," figuratively "put under obligation," from ob "to" (see ob-) + ligare "to bind," from PIE root *leig- "to bind" (see ligament). Main modern meaning "to make (someone) indebted by conferring a benefit or kindness" is from 1560s. Related: obliged; obliging.