The Rice University Neologisms Database

Breasts. Breasts. This is a metaphorical extension of the word ‘headlights.’ Most automobiles have two headlights, headlights are always on the front of cars, headlights on some cars are round, and headlights can be ‘flashed’. These similarities between breasts and automobile headlights allow the term ‘headlights’ to take on this meaning. The writer used this in a very informal context, and it was intended to be read primarily by college-age friends, so a slang word was appropriate.