How do I prevent xfce terminal from grabbing the Alt key?

[Mike Massonnet]

Hello,

So about the Alt key. I tested Xterm and indeed Alt+<letter> prints
some other letters, but this simply is a feature for Xterm and other
variants. Regarding GTK+ applications, there is nothing you can do
about it (well never say never), it is the default behavior including
VTE.

This includes all the Xfce applications, GNOME applications, even Chomium, etc.

GTK+ applications have something other to include special characters:
Ctrl+Shift+u. This will allow you to enter a UTF-8 unicode character
by its value. For example 2611 => ☑, or BF => ¿, etc.

You can always ask on the gtk mailing list, or look for help around
X11 oriented forums, mailing lists, since it looks like you are out of
luck here. As of a last hint, if you don't use Xmodmap, it may be
worth for you to look around the "Compose key", since this X11 feature
will work for any application.

http://sivers.org/compose-key

Kind Regards
-- 
Mike