From the manual pages on
Event Designators An event designator is a reference to a command line entry in the his‐ tory list. Unless the reference is absolute, events are relative to the current position in the history list.
! Start a history substitution, except when followed by a blank, newline, = or (. !n Refer to command line n. !-n Refer to the current command minus n. !! Refer to the previous command. This is a synonym for `!-1'. !string Refer to the most recent command preceding the current position in the history list starting with string. !?string[?] Refer to the most recent command preceding the current position in the history list containing string. The trailing ? may be omitted if string is followed immediately by a newline. ^string1^string2^ Quick substitution. Repeat the last command, replacing string1 with string2. Equivalent to ``!!:s/string1/string2/'' (see Mod‐ ifiers below). !# The entire command line typed so far.
All the others make sense, but I can't figure out any usage for
!# from the context. What does it do and how is it usually used?