Why is the ninth month called September?
1756, probably imitative of the sound made by something brushing against or through something. Related: Swished; swishing.
1820, from swish (v.); sense of "effeminate homosexual" is 1930s in homosexual slang, probably from notion of mincing motion.
To move, walk, speak, etc, in the manner or presumed manner of effeminate male homosexuals (fr homosexuals)
[perhaps fr the swinging movements of the hips in a mincing walk; perhaps fr swish2]