$ curl cheat.sh/
# xkill
# Kill a window interactively in a graphical session.
# See also `kill` and `killall`.
# More information: <https://www.x.org/releases/current/doc/man/man1/xkill.1.xhtml>.

# Display a cursor to kill a window when pressing the left mouse button (press any other mouse button to cancel):
xkill

# Display a cursor to select a window to kill by pressing any mouse button:
xkill -button any

$
Follow @igor_chubin cheat.sh tldr