I found this rare word — meaning effeminate — this week in S M Stirling’s book The Sky People, though he spells it slightly differently: “The muliebrous features could have been man or woman or creature from the stories his grandmother had told.”
His spelling has been used by others but mine is that in the Oxford English Dictionary, which has just one example, from 1652. It’s not quite that rare, though one has to search around for instances. It appeared in an article, The Industrial Value of Woman (a title that would today raise eyebrows) in The North American Review in 1882, in which the author wrote of a muliebrious or over-feminine woman.
Its companion adjective is muliebral, characteristic of women or womanhood, which lacks the other’s negative implications and which featured in the magazine The World & I in 1995: “Muller and Gillis represent the vibrant and irresistible muliebral force that has been weaving its way through the dance world for the last twenty years.”
Both derive from the classical Latin muliebris, womanly, which is from mulier, a woman. The latter is also the source of the even rarer and long obsolete legal term mulier that describes a child born in wedlock and so legitimate, and of the rather more common muliebrity, womanhood or femininity.