I've heard that there is no known verb for purring (of a cat) in Greek. Is that true? Any qualified guess as to what verb a Greek might have used?
Indeed, I cannot find any hits for "purr" in the LSJ, nor any verbs containing the words "cat" or "cats" in their entries! Quite a shame.
But there are several onomatopoeic verbs for making a low growling sound, such as ἀρῥάζω (arrházō), "to go arrha". Arrha, with a long, extended trill, sounds fairly close to a purr.
For a usage example, here's Aelian:
τὸ μὲν γὰρ βρυχᾶται, μυκᾶται δὲ ἄλλο, καὶ χρεμέτισμα ἄλλου καὶ ὄγκησις [ἄλλου], ἄλλου βληχηθμός τε καὶ μηκασμός, καί τισι μὲν ὠρυγμός, τισὶ δὲ ὑλαγμὸς φίλον, καὶ ἄλλῳ ἀρράζειν.
So one type of animal roars, another bellows, some make whinnying sounds, others make braying sounds, others go bleh and meh, some like howling, some like barking, and still others like to go arrha.
(DNA V.51, translation mine.)
This is the sort of thing you just don't get in modern biology textbooks.