When the word was borrowed into Middle French, French speakers gave it both a singular form (cisoire) and a plural form (cisoires). The plural didn't refer to multiple cutting implements, however; it was modeled on the two blades of a single caesorium.

Which is correct scissor or scissors?

Is it scissors or scissor? Scissors are handheld shearing tools that are useful for cutting many things. They are almost always plural or referred to as a pair of scissors. Thus, the singular scissor is almost never used.

Why are pants plural?

From its inception in English, pants has been plural. It's a shortened adaptation of pantaloons, those tight-like leg coverings worn by pirates and Shakespearean characters. And in those eras of 300 or 400 years ago, before pants were a single entity, pants were actually two separate garments.