When something occurs on or in the direction of one side, it happens sideways. You can lean over sideways to see if your cat's hiding under the bed, or tip a teapot sideways to pour the last drops out.
Sideways works as an adjective or adverb to describe things that lean toward one side or at an angle. A sideways tilt to a motorcycle might look scary, but it's safe if the rider is experienced. You can also use this word figuratively for "indirect" or "unconventional." So you might describe a movie that approaches serious subjects from a sideways perspective, using comedy. Sideways dates from the mid-16th century.