ĵuri

The trap is that English swear does two unrelated jobs. Ĵuri is only the oath one: you swear to tell the truth, you swear loyalty, you swear you were nowhere near it. Swearing in the sense of bad language is a different root entirely, so li ĵuris never means he was rude.

Structurally it is flexible. It takes ke with a clause, an infinitive, or a plain object such as ĵuri fidelecon, and the person you swear to takes al. The noun is ĵuro, an oath, and ĵurrompo is the breaking of one.