coat of mail

NOUN
12 LETTERS · STARTS WITH C

coat of mail is a valid 12-letter word worth 17 points in Scrabble and 20 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
C3O1A1T1O1F4M3A1I1L1
3 + 1 + 1 + 1 + 0 + 1 + 4 + 0 + 3 + 1 + 1 + 1 =17 PTS
DOUBLE WORD → 34TRIPLE WORD → 51
MEANING

What does coat of mail mean?