Why do Americans have no comprehension of the word "botty"?

It's just not a word that we use. What does it mean?

I'm not from the USA and I have no idea what the word means. In the UK it's a childish name for 'bottom' as in 'Mummy, come and wipe my botty.'

You bamboozle me.

is that a bot fly?

I am sure there are plenty of words Americans use which you'd be as equally unfamiliar with as they are of "botty".

When is this? Always?

Bots are flies that lay eggs on horses. http://equusmagazine.com/article/beatbotflies_071807