noun which may take the genitive case particle 'no'
Describes a person or thing that is completely unhelpful or fails to serve its purpose. Often used as a derogatory noun for a person.
彼は何もできない役立たずだ。
He is a useless person who can't do anything.
この道具は壊れていて役立たずだ。
This tool is broken and useless.