"A square has four angles."
Translation:Kvadrato havas kvar angulojn.
They tend to look that way when I draw them.
Actually, though, the etymology of the word is fairly simple. It derives from the Latin word quadratum, the same word which has also given us Quad (often the name for a space between buildings where students can relax at a school) which itself is an abbreviation of quadrangle, and that bane of many a math student quadratic, as well as a few other "square" words.
And yes, the ultimate root is quadri = 4