It might be confusing that this verb means both to lie and to be mistaken, any other synonyms? In Bailly, however, ψεύδομαι means mentir , to lie only, to be mistaken is ψεύδω. The English Greek Lexicon by Yonge gives ψεύδομαι as t be mistaken and to lie.
This is simplistic. You’d do better to use LSJ.