1. To require or constrain, as by law, command, conscience, or force of necessity.
2. To bind morally or legally, as by a promise or contract.
3. To place under a debt of gratitude for some benefit, favor, or service.
4. Etymology: "to bind by oath", from Old French obligier, from Latin obligare, from ob-, "to" + ligare, "to bind".

Main modern meaning "to make (someone) indebted by conferring a benefit or kindness" is from 1567; be obliged, "be bound by ties of gratitude" is from 1548. Obliging, "willing to do service or favors" is from 1632.