Lord knows I personally hate the word, but for those that don't know, the word generates from a German rifle made during WWII.
Sturmgewehr: "Storm Rifle" - the first assault rifle.
The word was storm rifle, which eventually translated into today's "assault rifle".
http://i188.photobucket.com/albums/z5/ethanrom_2007/mp44modelgun_1_ww2.jpg
I felt the need to share the history of that word.