underhand

Marked by deception.

“achieved success in business only by underhand methods”

With hand brought forward and up from below shoulder level.

“an underhand pitch”

“an underhand stroke”

Slyly and secretly.

“Mean revenge, committed underhand”

“oldline aristocratic diplomats underhandedly undermined the attempt...to align Germany with the Western democracies”

With the hand swung below shoulder level.