KanjiJLPT N1
Diagonal

diagonal, slanting, oblique

Mnemonic

An excess of broth ladled into a dipper makes it tilt at a diagonal angle. The overloaded dipper can't stay level—the excess weight drags one side down, and hot broth streams out in a diagonal cascade across the kitchen floor.
Additional thoughts
Picture yourself holding a giant ladle (dipper) filled way past the brim (excess). Gravity pulls the heavy side down, forcing the handle into a slant. That unavoidable tilt IS the diagonal.
Quick recall
An excess load in a dipper forces it to tilt diagonally.

Details

The keyword for 斜 is diagonal. This kanji refers to something that is slanting, tilted, or positioned at an angle rather than being perfectly vertical or horizontal. It conveys the idea of an oblique line or inclination, describing anything that deviates from a straight up-down or left-right orientation. It can also be used more abstractly to suggest something askew or not quite aligned with the norm.
On'yomi
しゃ
Kun'yomi
なな.め、はす