How to unset bash variable `!::=::\`?

Question

When I run printenv one of the returned variables is !::=::\. I noticed that the variable exists because docker does not work correctly with it set.

$ printenv | grep ::
!::=::\
$ docker stack deploy keycloak -c keycloak.yml
unexpected environment "=::=::\"

I am using git-bash on windows. The strange thing is that this variable only exists when I am using some terminal emulators such as ConEmu, Window terminal, Git-bash terminal while it does not exist when using VSCode's builtin terminal.

Why does this variable exist, and how do I remove it?

I tried the following methods with no success:

$ unset !::
bash: :: unrecognized history modifier
$ unset !::
$ unset !::
$ echo $!::
::
$ env | grep ::
!::=::\
$ printenv | grep :: | cat -A
!::=::$
$ export !::= 
bash: :: unrecognized history modifier
$ export !::=
bash: export: !::=: not a valid identifier
$ export $!::=
bash: export: ::=: not a valid identifier
$ set +H
$ unset !::
$ unset !::
$ env | grep ::
!::=::

The following works:

$ env -u \!:: env | grep::
$ env -u !:: docker stack deploy keycloack -c keycloak.yml
Creating network keycloack_default 
...

My environment:
OS: Windows with git-bash (MSys based)
Bash version: 5.2.15(1)-release

Answer 1

Here how I solved it. I created a .profile file that if it identifies a variable with ::, it starts a new bash without this variable.

$ cat $HOME/.profile
if env |grep -q ::
then
        exec env -u \!:: bash
fi

By the way, I tried to see where it comes from; The bash process is created by a mintty process whose parent is 1:

$ ps -p $$
      PID    PPID    PGID     WINPID   TTY         UID    STIME COMMAND
     2150    2149    2150      12536  pty1     1234897 15:34:37 /usr/bin/bash
$ ps -p 2149
      PID    PPID    PGID     WINPID   TTY         UID    STIME COMMAND
     2149       1    2149       9644  ?        1234897 15:34:37 /usr/bin/mintty

And this mintty process is created with this environment variable:

$ grep -z :: /proc/2149/environ
!::=::\

But since there isn't actually an init process (pid 1), I don't know how exactly this mintty process is created by git-bash, and why it starts with this environment variable. But anyway, the workaround above worked for me.

Answer 2

I have no idea why it exists, but you can unset it with env -u:

$ env | grep ::
!::=::
$ env -u \!:: bash -c 'env | grep ::'
$