discMṬR_2 is explained by Arab lexicographers as a fig. extension of MṬR_1: »marra ’l-farasu yamṭuru, vn. maṭr and muṭūr, and yatamaṭṭaru [vb. V] ‘The horse passed, or went, running vehemently, like the pouring of rain ’« (Lane 7-1885; my italics, SG). The vn.s however suggest that it may be an etymon in its own right.