E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
now means to take an oath, but the primitive sense is merely to aver or affirm; when to affirm on oath was meant, the word oath was appended, as I swear by oath. Shakespeare uses the word frequently in its primitive sense; thus Othello says of Desdemona
She swore, in faith, twas strange, twas passing strange.