threeside adjective 1noun 1forms 1 adjective1 •(nonstandard, rare) Having three sides; three-sided, triangular. noun1 •(nonstandard, rare) Any object or shape consisting of three sides; triangle.[...] let this base-point be indicated by Ax, while the sixth base-point will be indicated by C. Now [c*] contains a threeside formed by a, [...] — 1919 formsthreesides
