If someone swears, they use language that is considered to be rude or offensive, usually because they are angry.

If you swear to do something, you promise in a serious way that you will do it.

If you say that you swear that something is true or that you can swear to it, you are saying very firmly that it is true.

If someone is sworn to secrecy or is sworn to silence, they promise another person that they will not reveal a secret.