使えない
Tsukaenai
つかえない
Literally "cannot be used" — applied to things or people that are not useful. When directed at a person, it carries a very aggressive, demeaning tone. It is the kind of thing a toxic boss might say behind (or to) an employee's face. When used for objects (this computer is 使えない), it is simply descriptive. When used for people, it is one of the harshest casual insults in workplace Japanese.
Examples
あいつ、マジで使えないよな
That guy is seriously useless.
Talking behind a coworker's back at the office
うちの会社は、使えないやつに払う給料はないから
At this company, we do not pay salaries to useless people.
A dictatorial boss intimidating employees
このパソコン古すぎて本当に使えないわ
This computer is so old it is completely useless.
調べ物すらできないのか、お前本当に使えないな
You cannot even do basic research? You really are useless.