Etymology
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oblige (v.)

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.

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obligor (n.)

"person who binds himself to another by contract," 1540s, agent noun in Latin form from oblige.

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obligee (n.)

in law, "person to whom another is bound by contract," 1570s, from oblige + -ee.

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obliging (adj.)

of persons, dispositions, etc., "willing to do service or favors," 1630s, present-participle adjective from oblige. Related: Obligingly.

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obliged (adj.)

"bound by ties of gratitude," 1540s, past-participle adjective from oblige. Earlier it meant "be in bondage, be bound by (a promise, pledge, rule, etc.)," also "be liable for the payment of; be condemned" (mid-14c.).

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obligatory (adj.)

"binding in law or conscience, imposing duty, requiring performance of or forbearance from some act," c. 1400, obligatorie, from Old French obligatoire "creating an obligation, obligatory," and directly from Late Latin obligatorius "binding," from obligat-, past-participle stem of obligare "to bind, bind up, bandage," figuratively "put under obligation" (see oblige).

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obbligato (adj.)

musical instruction, used of accompaniments (especially by a single instrument, to a vocal piece), "so important that it cannot be omitted," 1724, from Italian obbligato, literally "obligated," from Latin obligatus, past participle of obligare "to bind" (see oblige). As a noun, "an accompaniment of independent importance," by 1817.

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obligate (v.)

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.

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obligation (n.)

c. 1300, obligacioun, "a binding pledge, commitment to fulfill a promise or meet conditions of a bargain," from Old French obligacion "obligation, duty, responsibility" (early 13c.) and directly from Latin obligationem (nominative obligatio) "an engaging or pledging," literally "a binding" (but rarely used in this sense), noun of action from past-participle stem of obligare "to bind, bind up, bandage," figuratively "put under obligation" (see oblige). The notion is of binding with promises or by law or duty.

The meaning "that which one is bound or obliged to do, especially by moral or legal claims a duty" is from c. 1600. That of "state or fact of being bound or constrained by gratitude to requite benefits, moral indebtedness," also is from c. 1600. Related: Obligational.

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*leig- 

Proto-Indo-European root meaning "to tie, bind." 

It forms all or part of: alloy; ally; colligate; deligate; furl; league (n.1) "alliance;" legato; liable; liaison; lien; lictor; ligand; ligament; ligate; ligation; ligature; oblige; rally (v.1) "bring together;" religion; rely.

It is the hypothetical source of/evidence for its existence is provided by: Latin ligare "to bind;" Albanian lidh "I bind," and possibly Middle Low German lik "band," Middle High German geleich "joint, limb."

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