Bitter-O's are a running joke among the Avengers Reassembled. It's actually quite an old joke but has been revived recently. It's usually directed at Nick Fury.
Bitter-O's are a play on the word 'Cheerios' or 'Fruity-O's' or some other generic cereal name. No one's sure who actually came up with the word, but Jay (then known as Jessica) was the first known one to use it. And it caught on from there.
The term 'Bitter-O's' is used to taunt someone who is bitter or grumpy ('Someone had a bowl of Bitter-O's this morning.') Usually it is associated with Nick Fury who's always in a bad mood.