The word šišlat is Czech and is usually translated as to lisp. However it refers to a form of speech impediment that involves substituting s /s/ sounds for sh /ʃ/ sounds. For example, slimák (slug) is pronounced šlimák. This isn’t quite the same as a lisp in English, which usually involves replacing s /s/ sounds with th /θ/ sounds.

Can you think of a good English version of šišlat?

I’ve come up with “to shish”.