Antonyms of adj sideways

1 sense of sideways

Sense 1
crabwise, sideways -- ((of movement) at an angle)

INDIRECT (VIA perpendicular, oblique) -> parallel -- (being everywhere equidistant and not intersecting; "parallel lines never converge"; "concentric circles are parallel"; "dancers in two parallel rows")
INDIRECT (VIA oblique, parallel) -> perpendicular -- (intersecting at or forming right angles; "the axes are perpendicular to each other")

Similarity of adj sideways

1 sense of sideways

Sense 1
crabwise, sideways -- ((of movement) at an angle)
       => oblique (vs. parallel) (vs. perpendicular) -- (slanting or inclined in direction or course or position--neither parallel nor perpendicular nor right-angled; "the oblique rays of the winter sun"; "acute and obtuse angles are oblique angles"; "the axis of an oblique cone is not perpendicular to its base")

Synonyms of adv sideways

4 senses of sideways

Sense 1
sideways, sideway, sidewise -- (with one side forward or to the front; "turned sideways to show the profile"; "crabs seeming to walk sidewise")

Sense 2
sideway, sideways, sidewise -- (from the side; obliquely; "a picture lit sideways"; "scenes viewed sidewise")

Sense 3
sideways, sideway, sidewise -- (toward one side; "the car slipped sideways into the ditch"; "leaning sideways"; "a figure moving sidewise in the shadows")

Sense 4
sidelong, sideways, obliquely -- (to, toward or at one side; "darting eyes looking sidelong out of a wizened face")