Вынесу, пожалуй, из комментов, потому что это и в самом деле любопытно.
Вот что говорит нам "Большой Оксфордский словарь" (существенное выделено мной):
читать дальшеmonkey, n.
(ˈmʌŋkɪ
[Of uncertain origin.
The MLG. version of Reynard the Fox (1498) has (only once, l. 6161) Moneke as the name of the son of Martin the Ape; and early in the 14th c. the same character is mentioned as Monnekin (v.r. Monnequin) by the Hainaulter Jean de Condé in Li Dis d'Entendement (Scheler) 853 (the passage is also printed by Chabaille as a ‘branche’ of the Roman du Renart). As the name does not occur in any other version of Reynard, the Eng. word can hardly be derived from the story. But it is not unlikely that the proper name may represent an otherwise unrecorded MLG. *moneke, MDu. *monnekijn, a colloquial word for monkey, and that this may have been brought to England by showmen from the continent. The MLG. and MDu. word would appear to be a dim. (with suffix -ke, -kijn: see -kin) of some form of the Rom. word which appears as early mod.F. monne (16–17th c.), It. monna (earlier mona), Sp., Pg. mona, mod.Pr. mouno female ape (a masc. mono occurs in Sp. and Pg.), whence the diminutive forms, early mod.F. monine, It. monnino and †monicchio (Florio). The origin of the Rom. word has not been discovered.]
I.I The simian animal, and transferred uses.
1. a.I.1.a In its widest application, an animal of any species of the group of mammals closely allied to and resembling man, and ranging from the anthropoid apes to the marmosets; any animal of the order Primates except man and the lemurs. In a more restricted sense, the term is taken to exclude the anthropoid apes, and the baboons; in popular use associated chiefly with the greenish long-tailed species having cheek-pouches, often kept as pets.
By some writers, the word ape is used to express the wider of the two senses above explained, and monkey is confined to its narrower application. Others employ monkey as the wider term, restricting ape to the tailless and especially the anthropoid ‘monkeys’.
ape, n.
(eɪp)
[OE. apa m., ape f., cogn. w. LG. ape, Du. aap, OHG. affo m., affe f., MHG. affe, ON. api (Sw. apa). Prob. an adopted word in OTeut.; cf. OIr. apa, Wel. epa; Slav. op-, in Old Boh. op, Boh. op-ec, Slovak op-itza.]
1.1 An animal of the monkey tribe (Simiadæ before the introduction of ‘monkey’ (16th c.), the generic name, and still (since 1700) sometimes so used poetically or rhetorically, or when their uncouth resemblance to men and mimicry of human action is the main idea (due to reaction of the vb. ape upon the n. whence it was formed).
2. a.2.a spec. A member of the Simiadæ, having no tail nor cheek-pouches; including the gorilla, chimpanzee, orang-outan, and gibbons. Короче: слово ape более древнее и общегерманское (ср., например, нем. Affe), а слово monkey завелось изначально чуть ли не из одного-единственного упоминания в "Романе о Лисе" (примерно так, как если бы у нас лису стали официально именовать "патрикеевной"), и происхождения оно темного. Но при этом слово monkey употребляется, в том числе, и для обозначения "обезьян" в целом, включая человекообразных (как в русском). Но в самом прямом смысле monkey - это какая-нибудь мартышка.