false claim → ψευδής ισχυρισμός, αναληθής ισχυρισμός
unfounded accusation → αβάσιμη κατηγορία
A false accusation is a claim or allegation of wrongdoing that is untrue and/or otherwise unsupported by facts. False accusations are also known as groundless accusations or unfounded accusations or false allegations or false claims. They can occur in any of the following contexts:
False accusation - Wikipedia