not have a leg to stand on
It has no basis. It is unreliable.
(Modern Hebrew following the expression 'there is substance to the matter')
Translation
Expression (Hebrew)
אֵין לָזֶה רַגְלַיִם
Definition (Hebrew)
אֵין לְזֶה בִּסּוּס. אֵינֶנּוּ אָמִין.
(הָעִבְרִית הַחֲדָשָׁה בְּעִקְבוֹת הַבִּטּוּי "יֵשׁ רַגְלַיִם לַדָּבָר")