I know merci is thank you but I have definitely heard some French people say (phonetically) mercish (i.e with a shesh on the end of merci). I have asked people about this and they seem to deny it! Is it just a dialect? I have heard it both in Northern France and down in the SW.
Sometimes people will say "mercish", "mershi" or "merki" instead of merci. It's a wrong pronunciation on purpose, to be funny, alleviate the mood or to be less formal than a straight "merci".
That's in no way specific to the word merci, but yes, you're right, some people pronounce a slight hiss and the end of words ending in -é(e) or -i(e).
I'm unsure it's really localized, though some regions can be more prone to that particular oddity.
You must have heard «mercikes». Those are "little thank yous" not in French but in Flemish, common here in Belgium.
Have heard “mersheesh” often in the Lot Valley near Cahors. I was told it was frowned upon as uneducated—similar to “warsh” for “wash” or the use of “ain’t” in the American South and Midwest.
"merchiche" peut être utilisé par un humoriste qui simule un lusophone, avec tout un environnement de phonations ibériques.
C'est un mot plus sensibles aux accentuations (mêêêêêêrci beaucoup) qu'aux différents parlés locaux.
Toutefois cela peut faire genre (attitude argotique pour paraître 'dans le vent', pour être 'in') de torturer les prononciations habituelles.