Green’s Dictionary of Slang

1. very drunk.

1812		Vaux Vocab. of the Flash Lang.
1834		‘The Corinthian’s Diary’ Museum of Mirth 59/2: Sunday got floored in groggy plight.
1838		‘Alfred Crowquill’ Seymour’s Humourous Sketches (1866) 44: ‘You’re drunk — drunk, Sarah, drunk!’ ‘On’y a little elevated, Jack.’ ‘Elevated! — floor’d you mean.’ ‘Vell; vot’s the odds as long as you're happy?’.
1933		‘William Juniper’ True Drunkard’s Delight 225: Our tippler may further be [...] floored.
1943		J.A.W. Bennett ‘Eng. as it is Spoken in N.Z.’ in AS XVIII:2 Apr. 89: A thoroughly drunk man is stonkered, floored, stunned.
1978		(con. 1963) J. Carroll The Basketball Diaries 27: A ‘spiller’ is the guy who wants to say he had as much as everyone else but knows he’d be floored if he did, so he’s always sneaking over to the window or making fifty trips to the john and spilling half or more of his drink away.

2. exhausted.

1869

T. Taylor New Men & Old Acres 8: Lil.: I am floored after the ball last night.

3. (US drugs) heavily intoxicated by a drug.

2001

Newark Advocate (OH) 21 Oct. 5A/4: Floored: to be too drugged to dance.