How much it is common?
Do Americans understand what it means?
This is very common in UK and Raymott is correct
It's not a word I'm familiar with, and I'm a lifelong American from the Midwest.
I've never heard this either. Is it common in the UK? A "bog" is a marshy place you grow cranberries. Is it a toilet in the UK? In Australia?
'The bog(s)' was a common word for the lavatories among boys when I was at school in the fifties and sixties; bogroll was provided, if one was lucky. It was still being used by schoolboys in the nineties; I don't know if it is common today.
Born and raised in USA, never heard this.