bash (esc dash) key combo?
 

I accidentally typed <esc> then a dash "-" and my command prompt says:

(arg: -1)

as if it were in a special mode. Can someone tell me the name of this so I may look it up? "bash escape dash" doesn't return any worthwhile results :(

Numeric Arguments

man bash, then search for "M--"

That meant nothing to me, so I Googled for "bash numeric arguments" and found more info.