Green’s Dictionary of Slang

an error .

1940		(ref. to 1919+) M. Marples Public School Slang 80: float (Eton, 1919+), a faux pas: also as a verb, to make a faux pas, drop a brick.
1956		N. Mitford Noblesse Oblige (1980) 87: I mean slang in the sense of a tribal patois (e.g. [...] float, a mistake).