Yeah, that's right. STFU. While this does not have a direct translation into Italian, nor is it a common marking in printed music, somehow it seems to be a common penciled in notation in the first trombone music of the AWCB.
"STFU" is a very nuanced marking. It does not merely mean to play quietly, but it is usually used to mark something that one feels should be played loudly, but really isn't supposed to be.
(Some part of me wonders... in twenty years, will someone else be handed this music and raise their hand and ask the conductor "I have 'stfu' penciled into my part, what does that mean?")