Word for the state/condition of feeling that one's life is meaningless?
If someone is depressed you can say, "We knew about his depression." How about the feeling of meaninglessness? I think you can't say, "We knew about his meaninglessness" (since it's not a state or condition).
Do you have angst, ennui, or weltschmerz?
I'd guess ennui. From the OED:
A feeling of listlessness and dissatisfaction arising from a lack of occupation or excitement
Example:
he succumbed to ennui and despair
Perhaps anomie is the word you are looking for:-
- Alienation and purposelessness experienced by a person or a class as a result of a lack of standards, values, or ideals [American Heritage Dictionary via the Free Dictionary]