from medieval or modern Latin pseudōnymus (from Greek ψευδώνυµος under a false name, falsely named) + -ous
1706 - The New World of English Words: Or, a General Dictionary, Edward Phillips;
"Pseudonymous, that has a counterfeit Name"
From: Athenæ Britannicæ
Or, A Critical History of the Oxford and Cambridge Writers and Writings
By Myles Davies
Part I. 1716
The Preface, P. 8