B2to promise or say firmly that you are telling the truth or that you will do something or behave in a particular way: I don't know anything about what happened, I swear (it).[+ (that)]You might find it difficult to believe, but I swear (that) the guy just came up to me and gave me the money.informalShe swore blind(= promised definitely)(that) she didn't know what had happened to the money.[+ to infinitive]New gang members must swear to obey the gang leaders at all times.In some countries, witnesses in court have to swear on the Bible.I swore an oath to tell the truth, the whole truth, and nothing but the truth.A few of us knew what was going to happen, but we were sworn to secrecy (= we were made to promise to keep it a secret).I think his birthday is on the 5th, but I wouldn't/couldn't swear to it (= I am not completely certain about it).Making and breaking promises and commitments