Green’s Dictionary of Slang

long-eared adj.

1. stupid.

1812		W. Tennant Anster Fair III xl 66: The long-ear’d lubbards in an even line / Then sat awaiting [...] When James’s herald should y-twang the sign.
1832		Satirist (London) 11 Nov. 365/2: That tough old tory, Sir Charles Wetherell, what a wonder in his way! His ‘blethering’ betrays the donkey, his obstinacy the mule. This happy country, indeed, is peculiarly blessed with fine specimens of the long-eared tribe.
1834		Ely’s Hawk & Buzzard (NY) 15 Mar. 2/1: The totally untenanted pericranium of this long-eared member of the human family .
1836		Westmorland Gaz. 14 May 2/3: We have been too long in harness to be deceived or misled by any long-eared brute.
1837		R.P. Robinson Life and Conversations 5: The public is a d----d, long eared ass, Sir!
1858		Yorkville Enquirer (SC) 22 Apr. 4/2: We do not wish to pollute our columns with such trash [...] snaggle-tooth — box-ankled — goggle-eyed — nigger-lipped — soft-headed — long-eared [etc].
1895		Truth (Sydney) 12 May 4/6: To Surgeon Lea the Naval Ass plainly said, ‘You are charged with insubordination’ [...] To Assistant-Paymaster Coulton same long-eared luminary said, ‘You are charged with embezzlement’.
1913		Day Book (Chicago) 18 Apr. 21/2-3: Another human being who acts like the animal with the long ears [...] the longeared professor.
1914		C. Holme Lonely Plough (1931) 162: The milk was the best in the county, as anybody but a long-eared sarsparilla raised on barley-water and lemon wouldn’t need telling.
1959		J. Thompson Getaway in Four Novels (1983) 20: The bombs, you long-eared jerk! Any commotion.

2. eavesdropping.

1956

‘Charles Raven’ Und. Nights 174: Some long-eared grass overheard and passed it on to the bogies.

In compounds

long-eared gentleman (n.) (also long-eared quadruped)

a donkey.

1826		Pierce Egan’s Life in London 8 Jan. 399/3: Tom would enact deputy donkey-driver, and urge Jane’s long-eared quadruped along the cliffs towards Ramsgate.
1838		‘Paul Pry’ Oddities of London Life II 269: The bench having expressed a wish to see the animal, the long-eared gentleman trotted into the office.