lapsus plumæ

The story

First attested in 1844; Latin: lāpsus (“a slipping”; in the plural lāpsūs, “slippings”, the nominative plural form of lāpsus) + plūmae (“of the feather or plume”, the genitive singular form of plūma, “feather”, “plume”) = “a slipping of the feather” ≈ “a lapse of the plume” ≈ “a slip of the quill”; compare lapsus linguae and the English-coined French phrase nom de plume.