Halakhah for Shevuot 49:5

To not swear in vain: That we not swear pointlessly, as it is stated (Exodus 20:7), "You shall not take the name of the Lord, your God, in vain." And the notion of pointlessness has four angles: [The first is,] for example (Shevuot 29a), that he swears about something known [to be true] that it is not so, like swearing about a pillar of marble that it is a pillar of gold. And so [too], anything like this. The second angle is, for example [Talmud Yerushalmi Shevuot 3:8], that he swears about what is known to be so, like about a stone that it is a stone and about a tree that it is a tree, and all that is like this. The third angle is that he swears to negate this commandment or the commandments that God, blessed be He, commanded us; as this is also completely pointless, since it is not in his hand to swear [to negate] that which God has already obligated him - and it is like the one who swears about something known that it is not so. The fourth angle is that he swears to do something that he does not have the power to do; for example (Shevuot 25a) that he will not sleep for three consecutive days, or that he will not eat for seven consecutive days. And so [too], anything like this.