Backdoor bragging is when you sneak something wonderful about yourself in everyday conversation.
30 Rock Episode Dialogue:
Kenneth: The personal essay is way harder than I thought, cause it's not in my nature to brag on myself.
Jenna: Not even a back door brag?
Kenneth: What's a back door brag?
Jenna: Backdoor bragging is sneaking something wonderful about yourself in everyday conversation. Like when I tell people it's hard for me to watch American Idol cause I have perfect pitch. You try it.
Kenneth: Oh, it's hard for me to watch American Idol because there's a water bug on my channel changer.
This comes back later in a talk with Liz:
Jenna: People always underestimate my instincts because of my good looks.
Liz: This is no time for back door bragging!