From ! + 1, from the fact that usually the same key is used for both symbols. Shift + 1 = !. This indicates that the person released Shift at some point, thus beginning to type 1.
!1
- (humorous) Alternative form of !! (dramatization of the ! punctuation)
1999 July 1, Simon Maguire, “Re: video about Nirvana”, in alt.music.nirvana[1] (Usenet; in English), message-ID <7lghei$nnd$5@nclient5-gui.server.virgin.net>:You'd all burn and go to hell!!!!!!!!!!!11111
