Adjectives

(s)askance, askant, asquint, squint, squint-eyed, squinty, sidelong(used especially of glances) directed to one side with or as if with doubt or suspicion or envy“her eyes with their misted askance look” (Elizabeth Bowen), “sidelong glances”
(s)lateral, sidelongsituated at or extending to the side“the lateral branches of a tree”, “shot out sidelong boughs” (Tennyson)
(s)sidelonginclining or directed to one side“moved downward in a sidelong way” (Bram Stoker)

Adverbs

(r)sidelongon the side“the plow lay sidelong on the ground”
(r)sidelongwith the side toward someone or something“seated sidelong to the window” (Nathaniel Hawthorne)
(r)sidelong, sideways, obliquelyto, toward or at one side“darting eyes looking sidelong out of a wizened face”