Rashi on Exodus 20:7/רש"י על שמות כ׳:ז׳

1לשוא IN VAIN — for no valid reason, idly. What is a שבועת שוא, an oath taken for no valid reason? If one takes an oath declaring something, the nature of which is evident, to be different from what it is: e. g., swearing about a stone pillar that it is of gold (Shevuot 29a).