Free dictionary Idioms tells the story: http://idioms.thefreedictionary.com/say+uncle
to admit that you have been defeated
Usage notes: In children's fights, a child being held down had to say 'uncle' before being allowed to get up.
I was good at refusing to say Uncle....my counselor told me more than once that I need to pick my battles and concede defeat sometimes. He told me to lose my Do-or-die Attitude. I looked at him very calmly and replied, "Without that attitude, I would have died." He thought about the experiences I finally was able to tell him and he agreed, I would have died. I am looking at different battles and I am selecting one or two to concede defeat and let myself fail.
Do you know what? I didn't die because I failed...hmmmm who knew? One of the stranger struggles I endure is knowing when to let something go. When to walk away from a fight. I loved the quote, "You don't have to join every argument you are invited to." Walk away. Many battles are not worth fighting. Save my energy for the things that are worth fighting for.