underhand

adjectives

with hand brought forward and up from below shoulder level
"an underhand pitch"; "an underhand stroke"
antonym
marked by deception
"achieved success in business only by underhand methods"

adverbs

slyly and secretly
"Mean revenge, committed underhand"- John Donne; "oldline aristocratic diplomats underhandedly undermined the attempt...to align Germany with the Western democracies"- C.G.Bowers
with the hand swung below shoulder level
"throwing a ball underarm"