What about "naff?"
Depending upon the context it means that something is no good, poor quality, a not-very-good idea or approach, badly executed, etc. Comparable to "duff" in many ways. For example:
"I had that car too, but it was a bit naff."
(Poorly built, unreliable, badly designed.)
"He thought he could manage it by himself. That was a naff idea."
(A duff or stupid idea, not well planned, unlikely to succeed.)
It can also be a bowdlerized term used in phrases such as "Naff off" or "Naff all," as popularized by the BBC TV show
Porridge in the 1970s.