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A primate, typically with a long tail and often used to refer to various species of monkeys.
Affen leben oft in tropischen Wäldern.
Monkeys often live in tropical forests.
In German, 'Affe' generally refers to monkeys with tails, while 'Ape' (without a tail) is called 'Menschenaffe'.
Remember that German nouns are always capitalized, so 'Affe' is always written with an uppercase 'A'.
The word 'Affe' is often used in idiomatic expressions, such as 'Affe machen' (to monkey around).
From Middle High German 'affe', from Old High German 'affo', possibly related to Latin 'apus' (bee) due to the similarity in sound.
In German, 'Affe' is capitalized as it is a noun. It is often used colloquially to refer to someone who is foolish or acts like a monkey.