E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Swear by my Sword (Hamlet, i. 5)
that is, by the cross on the hilt of my sword. Again in Winters Tale, Swear by this sword thou wilt perform my bidding (ii. 3). Holinshed says, Warwick kisses the cross of King Edwards sword, as it were a vow to his promise, and Decker says, He has sworn to me on the cross of his pure Toledo (Old Fortunatus).