Full of guile; treacherously deceptive.
Sly, dishonest, corrupt, cheating.
Done by moving the hand (and arm) from below.
Insincere; sarcastic.
An underhanded compliment is actually criticism.
Secret; surreptitious.
His underhanded trick backfired and he was disqualified.
Understaffed.
Devious people, collectively.
In an underhanded manner.