1520s, of horses, "to lead to a curb," from curb (n.). Figurative sense of "bend to one's will, hold in check" is from 1580s. Related: Curbed; curbing.
late 15c., "strap passing under the jaw of a horse" (attached to the bit of the bridle and used to restrain the animal), from Old French courbe "curb on a horse" (12c.), from Latin curvus, from curvare "to bend," from PIE root *sker- (2) "to turn, bend." The same word was used late 14c. in the sense of "a hump," and in Anglo-Latin as "curved or arched piece of timber" (late 13c.).
Meaning "enclosed framework" is from 1510s, probably originally with a notion of "curved;" extended to margins of garden beds by 1731; to "margin of joined, upright stones between a sidewalk and road" by 1791 (sometimes in this sense spelled kerb). Figurative sense of "a check, a restraint, that which holds back" is from 1610s.
also *ker-, Proto-Indo-European root meaning "to turn, bend."
It forms all or part of: arrange; circa; circadian; circle; circuit; circum-; circumcision; circumflex; circumnavigate; circumscribe; circumspect; circumstance; circus; cirque; corona; crepe; crest; crinoline; crisp; crown; curb; curvature; curve; derange; flounce (n.) "deep ruffle on the skirt of a dress;" krone; ring (n.1) "circular band;" ranch; range; ranger; rank (n.) "row, line series;" research; recherche; ridge; rink; rucksack; search; shrink.
It is the hypothetical source of/evidence for its existence is provided by: Latin curvus "bent, curved," crispus "curly;" Old Church Slavonic kragu "circle;" perhaps Greek kirkos "ring," koronos "curved;" Old English hring "ring, small circlet."
mid-14c., restreinen, "to stop, prevent, curb" (a vice, purpose, appetite, desire), from stem of Old French restraindre, restreindre "to press, push together; curb, bridle; bandage" (12c.), from Latin restringere "draw back tightly, tie back; confine, check" (see restriction).
From late 14c. as "keep (someone or something) from a course of action," hence "keep in check or under control, deprive (someone) of liberty by restraint" (1520s). Related: Restrained; restraining; restrainer; restrainable.
That which we restrain we keep within limits; that which we restrict we keep within certain definite limits; that which we repress we try to put out of existence. [Century Dictionary, 1902]