nano editor on OS X - How to type M-U for undo?

I am using nano 2.5.3 on OS X Yosemite, and I see commands at the bottom such as:

M-U Undo
M-E Redo

So far, I have not been able to figure out which key or keys that M is referring to. What would be M on OS X?

• There is no "UNDO" in OSX GNU nano 2.0.6.

• Meta (M-) symbol is the Esc key in OSX. E.g. to cut text with M-T, enter Esc+ T.

  The Main nano help text says:

  Escape-key sequences are notated with the Meta (M-) symbol and can be entered using either the Esc, Alt, or Meta key depending on your keyboard setup.

• Display the help text with all key combinations while in nano by entering ctrl+G.

  Hide the help text with the same key combination.

• Useful for navigating quickly:

  • ctrl+ Y / ctrl+ V -> Go one screenful up / down
  • Esc+ S -> Enable soft line wrapping
  • Esc+ M -> Enable Mouse support
  • Go to a line number

    • Esc+ N -> Enable line numbering

    • Esc+ _ -> go to a line

      You'll have to type 3 keys for that: Esc+ Shift + -

• FYI: In debian's GNU nano 5.4, enter Esc+U on an OSX keyboard to "UNDO".