Environment variable named !:: (exclamation mark + colon + colon)

[tzccinct]

Hi,

I have found an environment variable that has a strange name `!::' 
(exclamation mark + colon + colon) on my terminal (both mintty and Tera 
Term). The value is also strange, `::\' (colon + colon + backslash).

$ env | sort | head -n 3
!::=::\
_=/usr/bin/env
ALLUSERSPROFILE=C:\ProgramData

Is this an expected environment variable on Cygwin? If so, what is this for?

Best regards,
tzccinct