My go-to resource, Etymology online, has plenty of information about "floor" as a noun, but not as a verb to mean "surprise" or even "knock over." I just want to know if it would be anachronistic to have a character from the 1920's use the term.
From the OED to floor
In various figurative uses; colloquial:
To confound, nonplus; to flabbergast, puzzle. In schoolboy slang, to be or get floored: to grow confused, be at a loss, fail, break down.
1830 S. T. Coleridge Table-talk The other day I was what you would call floored by a Jew.
Early use of the figurative to floor predates a 1920 character, and use as such would appear to be appropriate and grammatical.