"Not to brag"
When you say something you did that was super awesome, but don't want to be a total cocky dick about it, you say this. Just to let people know what you did was great, but you're not too arrogant about it.
Person 1: "Yesterday, I had a shitty day."
Person 2: "Yesterday, I had an amazing day. NTB."
Person 1: "... Damn."