Green’s Dictionary of Slang

certainly.

1876		St Louis Globe-Democrat 19 Jan. n.p.: The invitation [i.e. to drink] is usually greeted with such exclamations as ‘don’t care if I do,’ ‘cert,’ etc.
1899		J. Flynt Tramping with Tramps 361: ‘Why, you’re not going to smoke here?’ I said. ‘Cert,’ he crisply replied.
1906		C. M’Govern Sarjint Larry an’ Frinds 56: Oi no sooner axed him about grub dan he answers me, ‘Cert! Come along, pardner!’.
1914		E. Wittmann ‘Clipped Words’ in DN IV:ii 137: cert, from certainly. ‘May I have this book?’ ‘Cert, help yourself.’.
1937		A.E. Duckett ‘Truckin ’round Brooklyn’ in N.Y. Age 6 Feb. 7/1: The keeds of Brooklyn are cert going capitalistic.