Green’s Dictionary of Slang

(orig. US) to betray secrets.

1884		(con. c.1840) ‘Mark Twain’ Huckleberry Finn 234: I reckon I couldn’t a stood it all, but would a had to bust out and tell on our gang.
1995		Francis-Jackson Official Dancehall Dict. 7: [...] 2. to reveal, let the cat out of the bag.