-M is for mouse events.

List of Keys
Collected from this source file

C-<key>       Hold Control + Key
M-<key>       Hold Meta + Key

F1            F1
F2            F2
F3            F3
F4            F4
F5            F5
F6            F6
F7            F7
F8            F8
F9            F9
F10           F10
F11           F11
F12           F12

IC            Insert
DC            Delete
Home          Home
End           End
NPage         Next Page / Page Down
PageDown      Next Page / Page Down
PgDn          Next Page / Page Down
PPage         Previous Page / Page Up
PageUp        Previous Page / Page Up
PgUp          Previous Page / Page Up

BSpace        Backspace
BTab          Back Tab
Enter         Enter
Escape        Escape
Space         Space
Tab           Tab

Up            Up
Down          Down
Left          Left
Right         Right

KP*           Keypad *
KP+           Keypad +
KP-           Keypad -
KP.           Keypad .
KP/           Keypad /
KP0           Keypad 0
KP1           Keypad 1
KP2           Keypad 2
KP3           Keypad 3
KP4           Keypad 4
KP5           Keypad 5
KP6           Keypad 6
KP7           Keypad 7
KP8           Keypad 8
KP9           Keypad 9
KPEnter       Keypad Enter

MouseDown1    Mouse Down 1
MouseDown2    Mouse Down 2
MouseDown3    Mouse Down 3
MouseUp1      Mouse Up 1
MouseUp2      Mouse Up 2
MouseUp3      Mouse Up 3
MouseDrag1    Mouse Drag 1
MouseDrag2    Mouse Drag 2
MouseDrag3    Mouse Drag 3
MouseDragEnd1 Mouse Drag End 1
MouseDragEnd2 Mouse Drag End 2
MouseDragEnd3 Mouse Drag End 3
WheelUp       Mouse Wheel Up
WheelDown     Mouse Wheel Down