marmoset n.

late 14c., "small ape or monkey," from Old French marmoset "grotesque figurine; fool, jester" (late 13c.), perhaps a variant of marmote "long-tailed monkey, ape," then, as a term of endearment, "little child." It is said to be from marmonner, marmotter "to mutter, mumble," which is probably of imitative origin. Some French authorities suggest a derivation of marmoset from marmor "marble," as if "little marble figurine." The English word was applied from early 17c. specifically to a type of small squirrel-like South American monkey.