Xfce4 dictionary has Ctrl-C as means of quitting
Most GUI programs on Linux and BSD use Control-Q to exit or quit a program, and Control-W to close one window of an application.
In the case of Xfce4 dictionary that is launched from the panel, it responds to Ctrl-C, which would normally be used for the copy-text feature.
In the code (lib/gui.cc) there is a check as to whether we are running as a plug-in and this is what leads to Control C being used.
Search for:
dd->is_plugin ? GDK_KEY_c : GDK_KEY_q,
FWIW the window that pops up from the panel button being pressed does not appear to be a part of a plugin. It looks like a separate program.
I propose to change the menu keyboard shortcut to Control Q in all cases and to change the text to always say Quit rather than Close, if possible.
If that is not possible because of a GTK limitation (as perhaps alluded to in the comments) I would secondarily propose to change the key sequence to Ctrl-W.