Dear etaoin: My handy-dandy ten buck CD dictionary says
roots are "four" and "feet". So a table has four legs, a trapezoid has four sides (two parallel), and trapezius muscles that attach scapular to back are four sided.
I guess the "trapeze" circus performers use is four sided also, two rope sides, bar to hang on, ropes at top hitched to a fourth "side" of some sort.
trapezium
n.,
pl. 3zi[ums or 3zi[a 73!8 5LL < Gr trapezion, trapezium, lit., small table, dim. of trapeza, table, lit., four-footed bench < tra3, for tetra, FOUR + peza, foot; akin to pous, FOOT6
1 a plane figure with four sides, no two of which are parallel: see QUADRILATERAL, illus.
2 Brit., etc. (exc. Cdn.) var. of TRAPEZOID (sense 1)