clandestine, secret or furtive
The backstairs (stairs at the rear of a building).
Secret; surreptitious.
Done by moving the hand (and arm) from below.
Sly, dishonest, corrupt, cheating.
Insincere; sarcastic.
An underhanded compliment is actually criticism.
His underhanded trick backfired and he was disqualified.
Understaffed.
Devious people, collectively.
In an underhanded manner.