The area in your head where thoughts of a single person constantly reside.
When a person online has annoyed you so much or out argued you so much that you constantly think about them and how to insult them again and again, and you constantly post about them, then they have occupied your crawlspace, similar to that small area under a house, they have moved in and taken up residence in your brain causing you to think about them all the time.
Boy look at her in that swimsuit... Crawlspace Cooter
Or for people in Alabama: "Did you see that ewe at the livestock auction, talk about some Crawlspace Cooter."
