I already seem to hear cries from geometers that this isn’t at all a weird word, but one with a specific and useful technical meaning of a triangle having unequal sides.
Mathematically inclined or not, we all have three of them, since the word is also applied to each of a set of three muscles that enable us to bend our necks — they’re called that because of their irregular triangular shapes; another name for them is scaleni, singly a scalenus, which is the Latin word from which scalene derives.
But the origin is further back, in the Greek skalinos, uneven or unequal.
One of its rare appearances outside technical contexts is this:
Mr. Minns found himself opposite a yellow brick house ... with ‘a garden’ in front, that is to say, a small loose bit of gravelled ground, with one round and two scalene triangular beds, containing a fir-tree, twenty or thirty bulbs, and an unlimited number of marigolds”.
Sketches by Boz, by Charles Dickens, 1836.