Laudator Temporis Acti

The striking phrase "trousered apes," as a description of men, comes from C.S. Lewis' The Abolition of Man (1947). Duncan Williams borrowed it for the title of a book published in 1971.

Perhaps Lewis was influenced by Erasmus, Praise of Folly:

An ape is always an ape, even if it's dressed in purple.

Simia semper est simia, etiam si purpura vestiatur.

However accurate the phrase "trousered apes" might be when applied to modern men, it would not have been an accurate description of the ancient Greeks and Romans, who eschewed trousers as barbarian garb. In a collection of passages from ancient authors on trousers, I once wrote:

The Greeks didn't have a native word for trousers, so they borrowed anaxurides from Persian. The Latin word bracae (sometimes spelled braccae), whence English breeches, may also be a loan word from Gaul.

This isn't completely accurate. I recently came across a native Greek slang term for trousers, θύλακοι [thulakoi], the plural of θύλακος, whose primary meaning is "bag, sack."

The secondary meaning "loose trousers" occurs at Aristophanes, Wasps 1087. The old men of the chorus are relating how they defeated the Persians in battle (tr. Alan H. Sommerstein):

Then we pursued them, harpooning them through their baggy trousers.

εἶτα δ᾽ εἱπόμεσθα θυννάζοντες ἐς τοὺς θυλάκους.

This secondary meaning also occurs at Euripides, Cyclopes 182, in a description of the seduction of Helen by Paris (182-186, tr. E.P. Coleridge):

The sight of a man with embroidered breeches on his legs and a golden chain about his neck so fluttered her, that she left Menelaus, her excellent little husband.

ἣ τοὺς θυλάκους τοὺς ποικίλους
περὶ τοῖν σκελοῖν ἰδοῦσα καὶ τὸν χρύσεον
κλῳὸν φοροῦντα περὶ μέσον τὸν αὐχένα
ἐξεπτοήθη, Μενέλεων, ἀνθρώπιον
λῷστον λιποῦσα.

Besides anaxurides and thulakoi, there is a loan word for trousers listed in Liddell-Scott-Jones, s.v. σαράβαρα [sarabara], defined as:

loose trousers worn by Scythians, Antiph.201; also = Aramaic sarbālîn, LXX, Thd.Da.3.27 (cf. 21). (Prob. Persian shalvâr or shulvâr (braccae).)