xkill

Kill window by clicking

$ xkill

$ xkill -id [window_id]

$ xkill -frame

$ xkill -button any

xkill [-id windowid] [-button button] [-frame] [options_]

xkill forcibly closes X11 windows. After running, the cursor changes and the next clicked window is killed.
The tool sends a kill request to the X server. This is more forceful than asking the window to close normally.
Window manager frames can be targeted. Using -frame kills the window even if clicking the decoration rather than content.
Any mouse button can trigger the kill. By default, only button 1 (left click) works. The -button option changes this.
Pressing any key cancels xkill without killing a window. This provides an escape if started accidentally.
The tool is useful when applications become unresponsive and normal close methods fail.

-id WINDOWID

Kill specific window by ID.

Button to use (any, 1, 2, 3).

Include window frame.

Kill all matching clients.

X server display.

X11 only - doesn't work on Wayland. May lose unsaved data. Doesn't gracefully close applications. Some windows may resist.

xkill has been part of the X.Org utilities since the early days of X11. It provides a simple way to deal with misbehaving X applications.

kill(1), killall(1), xwininfo(1), xdotool(1)