I will sit this one out!
A and B doesn't get along very well. Therefore, C said, "They can't go on like this ! We have to do something to help....". D said, " I will sit this one out".What does "i will sit this one out" mean here? Thanks!

Question

I will sit this one out!
A and B doesn't  get along very well. Therefore, C said, "They can't go on like this !  We have to do something to help....". D said, " I will sit this one out".What does "i will sit this one out" mean here? Thanks!

Answer

I will sit this one out = I will not take part in this = I will not interfere.
Literally, this phrase to mean that the person will not take dance for one song at some party or dance.  
("Would you like to dance?"  "I'm a little tired, I think I'll sit this one out.")
In normal conversation, it means that the person will not involve themselves in some effort or struggle.