Usually pronounced "stef" when said aloud, which it hardly ever is, stf is a now rarely used abbreviation for scientifiction, chiefly employed today in its adjectival form, stfnal (pronounced "STEF-nel" or "STIF-nel"), which applies to anything science-fictional or part of the microcosm.

Stfnal has positive connotations, while skiffy is mildly negative and sci-fi denotes disapproval.

Other forms, such as stfsy for science fiction and fantasy, have fallen totally out of use, though you still see stfnist now and then.

from Fancyclopedia 2 ca. 1959
(Hogenmiller:Ackerman) Formerly the abbreviation for Scientifiction, now pronounced "stef" or in a number of ways which George Charters reviews below. (It is NOT pronounced "ess tee eff"!!!) It is now a short form for science-fiction, and in the form stfsy for science-fiction AND fantasy — i e the whole field of fantasy writing. Adjectival form stfnal means pertaining to science-fiction, or sometimes to fandom. "St(e)fnist" was (1) proposed by Speer as a substitute for "fan" on account of the undesirable connotations of the latter word, but has come to mean (2) any person interested in fantasyfans, pros, and mere occasional readers. Such folk have also been described as the stfnate, and rarely stfim.

Some fans wonder, "What's the diff?"
They don't care if you call it stf.
But some, whose noise would make you deaf
Vociferously yell for stf.
The non-fan says, "A lotta guff —
It's obviously only stf."
Myself, I laugh a scornful laugh;
I think the damn thing might be stf.
James White prefers to stand aloof,
But, pressed, he coyly murmurs "stf".
Bob Shaw just shrugs the matter off.
For all he cares, it could be stf.
And Walter Willis and his wife
Say "Erudition calls for stf."

Thus we can see the contradiction
In people's names for stf.
from Fancyclopedia 1 ca. 1944
(Hogenmiller:Ackerman) - Formerly the abbreviation for scientifiction, now pronounced [stef], and used as a short form for "science-fiction". It should not be pronounced [estief]!