Green’s Dictionary of Slang

(US) stupid.

1859		C. Mackay Life and Liberty in America 102: Amiable means ‘stupid.’ A member of the House of Representatives, and a most worthy man, was highly offended at hearing his friend called ‘amiable’ by an Englishman. He thought the phrase implied a reproach, or a sneer.
1886		Clarkesville Wkly Chron. (TN) 30 Oct. 2/3: We point out to that amiable young man the error of his way [...] The Leaf then proceeds to read the ‘amiable’ young man a leson.