One, two! One, two! and through and through
The vorpal blade went snicker-snack!
He left it dead, and with its head
He went galumphing back.
Vorpal is a nonsense word, but from the context of the poem, it is understood to be a blade capable of decapitation.
It entered the world of D&D meaning a magic sword with an increased tendency to decapitate one’s opponent, or to severe a limb.