An Etymological Dictionary of the German Language/Sturm
Sturm, m., ‘storm, tumult,’ from MidHG. and OHG. sturm, m., ‘tempest, fight’; comp. Du. storm, AS. and E. storm, and the equiv. OIc. stormr. From the common Teut. storm (sturm) are derived the Romance cognates, Ital. stormo, ‘concourse, encounter, quarrel,’ which proves the primitive use of the word in the figurative sense of ‘fight’ (E. stour is based on the corresponding OFr. estour). The Teut. root stur is a relic of the Aryan root ser (sṛ from stur?), to which Gr. ὁρμή, ‘attack, impact,’ Sans. root sṛ, ‘to stream, hasten,’ belong (for str from sr, see Schwester and Strom). Others prefer to regard the word as primitively cognate with Lat. sternere, ‘to throw down.’