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.
1626		T. Deloney The pleasant historie of Iohn VVinchcomb m.p.: Good morrow good Gossip: now by my truly I am glad to sée you in health.
1638		T. Heywood The vvise-woman of Hogsdon n.p.: I would not have it knowne to my Neighbours, that I come to a Wise-woman for any thing, by my truly.
1647		J. Cleveland A London-diurnall 35: And these Drum-Major oaths of Bulke unruly, May dwindle to a feeble By my truly.
1650		A. Cowley The guardian n.p.: Let me alone, I’le deal with no oath above gods fatlikins, or by my truly.
1662		M.W. & M.A. The marriage broaker 39: Now by my truly it doth ill beseem / Your son-ship to revile your father thus.
1677		W. Cavendish The triumphant widow 13: Codsh. [W]hat say you to by my truly? Cramb. ’Tis a childish Oath.
1683		E. Hickeringill The Trimmer 7: Your Whigg never swears, or if he does, ’tis some dwindling Oath, (as) by my truly; whereas your true Tory-Boy lets fly [with] damme, sink me, Hell and Damnation [...] and dam the Whiggs.
1697		J. Vanburgh The relapse 60: Nurse. Yes, by my truly, is he. Miss. O Lord! I'll go put on my Lac’d Smock, tho’ I’m whipt ’till the blood run down my heels for't.
1795		Jemima I 110: Part, repeated Rosina, yes, by my truly must we [OED].