agreement (n.)

late 14c., "to give consent, assent," from Old French agreer "to please, satisfy; to receive with favor, take pleasure in" (12c.), a contraction of the phrase a gré "favorably, of good will," literally "to (one's) liking," or a like contraction in Medieval Latin. The French phrase is from a "to," from Latin ad (see ad-) + Old French gre, gret "that which pleases," from Latin gratum, neuter of gratus "pleasing, welcome, agreeable" (from suffixed form of PIE root *gwere- (2) "to favor").

In Middle English it also meant "to please, gratify, satisfy," a sense preserved in agreeable. Of parties, "come to agreement; make a settlement," mid-15c.; the meaning "to be in harmony in opinions" is from late 15c. Of things, "to coincide," from 1520s. To agree to differ is from 1785 (also agree to disagree, 1792). Related: Agreed; agreeing.

late 15c., "refusal to agree or assent," from disagree + -ment. From 1570s as "difference in form or essence," also "difference of opinion or sentiments," perhaps a separate formation from dis- + agreement. From 1580s as "a falling out, contention." As "unsuitableness, unfitness," by 1702.