(Minneapolis Fantasy Society) - An outcry of surprise (and often disgust). "Hot Fout!" (sometimes "Hotfout!") is the superlative form, and more frequently has the connotation of pleasurable surprise.

It also illustrates the old saying about Americans being abysmally ignorant of languages other than English, since nobody apparently recognized French foutre, Latin futere, as English "Fuck".

In fandom it was apparently first used some time in the 40s by a (mundane) younger brother of Phil Bronson while in earshot of members of the Minneapolis Fantasy Society and from them spread quickly through fandom. It originally was an expression of utter disgust, but with time has mellowed to indicating mild impatience or grumpiness at being provoked.

It is largely obsolete today.

from Fancyclopedia 2 Supplement ca. 1960
"Hotfout" also has a non-derogatory use. And "fout" itself is often a verb, as in the jingle:
Fantasite's a fouty rag, fouty rag, fouty rag,
Fantasite's a fouty rag,
Fout on Fantasite!
from Fancyclopedia 1 ca. 1944
(MFS - Root of such words as "fouty" (a very vague adjective of derision) and Hotfout!" (an interjection expressing extreme joy or exuberance).