intransitive. To make a solemn declaration or statement with an appeal to God or a superhuman being, or to some sacred object, in confirmation of…

To take a solemn oath. rare.

transitive. To declare or promise (something) on oath, to swear solemnly. Chiefly with that, or with clause as object. Also occasionally intransitive…

used in various expressions with reference to the custom of using bread and salt to solemnize an oath, affirmation, etc.

= affidavit, n., esp. in to take one's davy (= ‘to take one's oath’).

To swear to.