It never occurred to me in essays that it was spelt “scissors”

Sissors makes me very uncomfortable

Why are the esses even there? It should be z's.
And why the hell isn't phonetically spelled phonetically??
Furthermore...who is the dick that put an "s" in tbe word lisp?!

Because it came from a language in which the c was pronounced

Yesc

Maybe you should actually look up the etymology.

Without the C it would be pronounced
Sis-sors