Aliasing fat_ass → invalid_tag
Link to alias

Reason:

Ambiguous. Could refer to big_butt or overweight.

Fat ass: a large buttocks
Fatass: an overweight person

Seriously though, it is a useless tag.

Seems to be 99% used for big_butt to me

Seems to be mostly tagged by 1 person.

Either way though, it seems like it would make enough sense to alias both this and fat_butt -> big_butt. That tag tends to get tagged with overweight anyways, so 1-2 of them every now and then shouldn't be much of an issue if it means that 95% of them go to the right place.