Cool story bro **sincere** *adj.* 1533, honest, straightforward; borrowed from Middle French *sincere,* from Latin *sincērus* sound, whole, pure, genuine, perhaps originally “of one growth,” not hybrid, unmixed (dissimilated by loss of *r* after *c* in earlier *sincrēros), from *sem-,* *sin-* one (cognate with Greek *heis, hén* one; see SAME) + the root of *crēscere* to grow; see CRESCENT.