A portmanteau of the words necessary and accessory.
Etymology: Unlikely original. Incorrectly misspelled as necessory sometimes.
1) An accessory that is necessary to the function of the object it accessorizes.
2) Used ironically to make it sound like the item is question is necessary for the continuation of all human existence. While in fact it is not.
1) "Those pedals are a necessessory to your bike."
2) "That new cell phone is a total necessessory. I will die with out it!"