Green’s Dictionary of Slang

1. to die.

1701		Farquhar Sir Harry Wildair IV ii: The Dog was soon gone, knock’d under presently.
1815		‘Hey for the Life of a Soldier’ in Vocal Mag. 2 Jan. 10: Blood and thunder, / Foes knock under, / Then huzza for a victory.
1843		L.H. Medina Nick of the Woods I iii: Tarnal death to the crittur; I’m the man to make him knock under!
1891		H.B. Marriott-Watson Web of the Spider 183: We were all as near knocking under as possible.
1898		H.B. Vogel Maori Maid 158: When father knocks under, me and my brother we’ll come into the business.

2. (Aus.) to surpass.

1832		Satirist (London) 21 Oct. 341/2: Lord PALMERSTON. has given up / His Conferencial thunder, / That roared abroad in Protocols / To knock the Dutchmen under.
1901		Bulletin (Sydney) 16 Nov. 14/4: It is all bunkum to say our darkies can’t graft. I’ve seen ’em, in N.S.W. quarries, knock white men under.
1908		G. Seagram Bushman All 31: If you don’t knock it [drink] off, it’ll knock you under.