Looking up words in the dictionary, I always encounter symbols like these in front of words.
[U. sing.] Distrust
I know [U] means it's an uncountable noun for sure. But what does the first one mean?
1)The dictionary is telling you that distrust can be used:
either as an uncountable noun [U]: Distrust is a bad thing.
or as a singular [sing] in reference to a particular instance of distrust: She now feels a distrust and suspicion towards him.
2)[U] Health: I doubt you may say "a health".