英语词源字典 - 在线词源网站

arcana (n.)[arcana 词源字典]: "hidden things, mysteries," 1590s, a direct adoption of the Latin plural of arcanum "a secret, a mystery," from neuter of adjective arcanus "secret, hidden, private, concealed" (see arcane). Occasionally mistaken for a singular and pluralized as arcanas because arcana is far more common than arcanum.[arcana etymology, arcana origin, 英语词源]
atomies (n.): 1590s, "atoms," also "diminutive beings," from atomy, from Latin atomi, plural of atomus (see atom), but taken as a singular in English and re-pluralized in the native way. Perhaps also in some cases a plural of atomy (from misdivision of anatomy).
cilia (n.): 1715, from Latin cilia, plural of cilium "eyelid, eyelash," perhaps related to celare "to cover, hide," from PIE root *kel- "to conceal" (see cell), but words for this part of the face can be tricky (see brow). It sometimes is pluralized in English, which is an error. Related: Ciliated; ciliary; ciliate.
effluvia (n.): Latin plural of effluvium. Sometimes mistaken for a singular and re-pluralized.
themselves (pron.): mid-15c. in northern dialect, standard from 1540s, alteration of Middle English tham-self, emphatic plural pronoun, also reciprocal pronoun (14c.); see them + self, with self, originally an inflected adjective, treated as a noun with a meaning "person" and pluralized. Displacing Old English heom selfum (dative). Themself returned late 20c. as some writers took to replacing himself with gender-neutral everyone, anyone, etc.