Green’s Dictionary of Slang

(Aus./US) to eavesdrop.

1912		D. Lowrie My Life in Prison 79: There’s a stool-pigeon tin-earin’ right behind us.
1914		Jackson & Hellyer Vocab. Criminal Sl. 83: tin ear [...] To eavesdrop; to listen impertinently.
1931		Irwin Amer. Tramp and Und. Sl. 189: Tin Ear. – To eavesdrop ; to listen with especial care.
1962		Ragen & Finston World’s Toughest Prison 821: tin ear – To eavesdrop; to listen with special care.