Green’s Dictionary of Slang

an eavesdropper.

1865		Leaves from Diary of Celebrated Burglar 18/2: Having no ‘earwiggers’ near, Joe commenced his story of entering the jeweler’s store.
1908		Sun. Times (Perth) 5 Jan. 1/1: A fight in a Fremantle Club has kept ear-wiggers in beer for a week.
1974		G.F. Newman You Flash Bastard 101: Sneed wondered at times how the Fly ever brought himself to use the phone, he was that paranoid about earwiggers.
2006		www.overheardindublin.com 23 June 🌐 (irate) girl on mobile: ‘.......so you’re telling me I can’t see you until after the world cup?’ Overheard by earwigger, Stephens Green.