Phrase used after a very indepth, overly detailed explanation, definition, anticdote, or nerdy personal admission is made. Can be used to describe yourself or others. Can be a diss or a tension breaker for realizing how nerdy you just came off.
Sue: "I used to read black beauty over and over again when I was a kid, we should put that on the list of the greatest books of all time."
Jane later after recounting the incident to Amy.
Jane: Nerd Alert! Could she love the book black beauty any more?!?
My biochemistry teacher was a super nerd. After she went off on a tangent for about fifteen minutes about some small detail she thought was so cool and she used to study, she realized she was at her nerdiest, and could only recover with exclaiming "Nerd alert!" in her defense and move on.