Green’s Dictionary of Slang

a mild oath, used to underpin the veracity of one’s statement.

1580		G. Harvey Two Letters in Works (Grosart) I 42: By my truely, I was neuer so scared in my lyfe.
1604		Webster Westward Hoe II i: Have you a new pen for me, master? for, by my truly, my old one is stark naugh.
1795		Jemima I 110: Part, repeated Rosina, yes, by my truly must we [OED].