Its when you are fucking a chick doggy style and you pull out your dick right before you are going to cum, place it on the top of her ass crack, then run a line of gunk
from the top of her ass to her clit just like a caulking gun. This of course would also apply to the upper lip, forehead, ear-to-ear across the eyes, ect.
"I just laid a smooth line of baby batter
on my girls ass crack with my cocking gun".
"That girls ass was so big I had to reload my cocking gun to finish the job".