Origin: Latin suffix -ary
Unnecessary has 2 different meanings across 1 category:
not necessary
"The extra luggage was unnecessary because we had plenty of space in our car."
Not needed or necessary.
"The extra layer of paint was unnecessary because the wall was already smooth and clean."
The word unnecessary comes from combining the prefix un- with necessary. It entered English simply by adding a negative meaning to an existing term that already described something needed or required.