let the cat out of the bag

VERB
26 LETTERS · STARTS WITH L

let the cat out of the bag is a valid 26-letter word worth 34 points in Scrabble and 37 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
L1E1T1T1H4E1C3A1T1O1U1T1O1F4T1H4E1B3A1G2
1 + 1 + 1 + 0 + 1 + 4 + 1 + 0 + 3 + 1 + 1 + 0 + 1 + 1 + 1 + 0 + 1 + 4 + 0 + 1 + 4 + 1 + 0 + 3 + 1 + 2 =34 PTS
DOUBLE WORD → 68TRIPLE WORD → 102
MEANING

What does let the cat out of the bag mean?