obligation n.

c. 1300, obligen, "to bind by oath, put under moral or legal obligation, devote," 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 tie, bind." Main modern meaning "to make (someone) indebted by conferring a benefit or kindness" is from 1560s.

1540s, "to bind, fasten, connect," the literal sense of the Latin word, now obsolete in English; 1660s in the main modern sense of "to put under moral obligation;" a back-formation from obligation, or else from Latin obligatus, past participle of obligare "to bind, bind up, bandage," figuratively "put under obligation" (see oblige). Oblige, with which it has been confused since late 17c., means "to do one a favor." Related: Obligated; obligating.