Why is the ninth month called September?
"to emit soft, rapid sounds," late 14c. (implied in rustling), of uncertain origin, perhaps imitative (cf. Middle Low German ruschen, Middle Dutch ruusscen, German rauschen "to rustle"). Related: Rustled; rustling. Meaning "steal" (especially cattle) first attested 1882, probably from earlier American English slang sense of "move about vigorously" (1844), perhaps a separate word, compounded from rush and hustle.
1759, from rustle (v.).
[origin unknown; perhaps fr rush plus hustle]