under splash 

The outcome of forced downward ejection of urine forcing loo water onto the underneath of the toilet seat.

Lesley you just caused under splash in the downstairs toilet.

under splash by MOI6943 June 11, 2022

underslash 

> _ < the thing in the middle there is a underslash, often mistaken for a "underscore"

www.myspace.com/underslash_
used to seperate words in website adresses

underslash by sfha;ob December 30, 2007

Undersplash 

The outcome of downward force of female urine, splashing back up to the underneath of the toilet seat

She was responsible for undersplash when seated on the toilet

Undersplash by Bvs1 June 12, 2022