In software development, 'TU' meant "stopped working at this point”. I’m a dinosaur, that was back in my mainframe days where if something my team was working on failed, a giant stack of fanfold paper would show up (typically thick enough that it came on a cart), and someone more experienced than me would flip through, see where things had gone wrong, draw a line across the page with 'TU’. I had to ask a coworker what that meant. Possibly unfortunately, although she showed no discomfort, it was a female coworker (who also happened to be a good friend).

She says "tits up"

Me: ???

She says "Bob, it *died* there - think about it like somebody in a casket - which way do their tits point?"

Me: "Oh... thanks!"

I moved into PC development a few months later, still kept hearing the expression. - I also learned that "MT" on a box in storage meant it was empty, which is also a two-letter acronym, but is way off topic here...