how exactly is this 'bottomless?'

She is not wearing a skirt nor pants, therefore, bottomless ^^

What takashi90 said.

Just throwing this out there, but if you're confused about a tag, please read the wiki by clicking on the question mark next to the tag or search for it in the Wiki section manually, then make a judgment on whether or not to add/remove it. Things will go much more swimmingly that way.