Learn English – What does “Swish” mean

'Skirt' is not sexist by itself (where 'sexist' means derogatory by sexual connotation). The word 'skirt' simply refers to a short dress, a garment these days that is worn mostly by women.

'Swish of her skirt' is not sexist by itself, where 'swish' ostensibly means a swaying back and forth movement. But the two words together evoke a bit of coquettish imagery, that the wearer might intentionally be moving her skirt provocatively.

A 'venomous swish of her skirt' is definitely sexist as it implies metaphorically that the 'swish of her skirt' (with the coquettish interpretation, is poisonous and therefore dangerous because of its female attributes, implying an out of the ordinary danger or tendentious difficulty based exclusively on the one-sided sexual characteristics of being female.

The phrase 'swish of her skirt' is not an idiom, but it is metaphorical. With 'venomous' it becomes very negatively intentioned.

So the phrase altogether is very sexist.

Joyce is famous for his neologisms and Kalipedia is one such. The entire sentence reads:

Kalipedia, he prophesied, would soon be generally adopted and all the graces of life, genuinely good music, agreeable literature, light philosophy, instructive pictures, plastercast reproductions of the classical statues such as Venus and Apollo, artistic coloured photographs of prize babies, all these little attentions would enable ladies who were in a particular condition to pass the intervening months in a most enjoyable manner.

The word is constructed from the Greek kali (meaning beauty, as in calligraphy) and paedia (meaning education) and could be construed to mean education through beauty or perhaps enrichment through beauty. Joyce (or his character, Mr. Mulligan) very likely derived the word from the Greek phrase, Kalos kagathos:

Kalos kagathos (Ancient Greek καλὸς κἀγαθός [kalos kaːɡatʰǒs]), of which kalokagathia (καλοκαγαθία) is the derived noun, is a phrase used by classical Greek writers to describe an ideal of personal conduct, especially in a military context. Its use is attested since Herodotus and the classical period. The phrase is adjectival, composed of two adjectives, καλός ("beautiful") and ἀγαθός ("good" or "virtuous"), the second of which is combined by crasis with καί "and" to form κἀγαθός. Werner Jaeger summarizes it as ”the chivalrous ideal of the complete human personality, harmonious in mind and body, foursquare in battle and speech, song and action”.