What does "grinning bitterly" mean?

Question

I daresay I was not very well at that time. I tottered about the streets -- there were various affairs to settle -- grinning bitterly at perfectly respectable persons.

Answer 1

"bitterly" almost always signifies that a person feels that they have been done wrong, but without any reason. The person in your sentences feels like the "respectable persons" are innocent people that the speaker feels have hurt him, though for no reason at all.

"grinning bitterly" is also a good example of consonance.

Answer 2

What it means is making believe that you are smiling or happy when you really aren't or smiling when you really don't feel like smiling.  Hope this helps.