Green’s Dictionary of Slang

mack n.¹

In exclamations

by the mack! (also by the mackin(s)! by the mackings! by the maskins!) [? SE by the Mass! or by Mary!]

a general oath (cf. holy mackerel! excl.).

1605		London Prodigall II ii: A by the mackins, good syr Lancelot .
1664		C. Cotton Virgil Travestie (1765) Bk I 59: By th’ Mack (quoth she) thou Trojan trusty. [Ibid.] Bk IV 90: And now this Swabber, by the Maskins, / Thunders up Dido’s Gally-Gaskins.
1667		Dryden Sir Martin Mar-all IV i: By the Mackings, I thought there was no good in’t.
1694		J. Eachard (trans.) Plautus’s Amphitryon I i: By the Mackins, I believe Phebus has been playing the Good-Fellow.
1731		C. Coffey Devil to Pay I iii: By the Mackin, she’s drunk, bloody drunk.
1854		Examiner 24 June 6/2: By the merry-maskins, Peggy, you’re a good judge.
1857		Manchester Times 26 Dec. 2/1: By the maskins, Lord.