Synonyms of underhandedly

2 Syllables
slyly, quietly, hush-hush
3 Syllables
stealthily, privily, secretly, untruly, falsely, covertly, basely, unduly, faithlessly, roguishly
4 Syllables
insidiously, furtively, privately, unobserved, untruthfully, mendaciously, deceivingly, obscurely, illegally, personally, traitorously
5 Syllables
surreptitiously, confidentially, clandestinely, intimately, falseheartedly, malevolently, indecorously
7 Syllables
disproportionately