Green’s Dictionary of Slang

(US) someone or something judged to be worthless, insignificant or boring.

1882		G.A. Sala America Revisited I 50: The chances are that his indignant hearers will vote him a ‘cold potato’ and ‘run him out’.
1893		P.H. Emerson Signor Lippo 84: Oh! I knows ’em all and can recon ’em up, from a shack stone to a cold ’tater.
1901		‘Hugh McHugh’ John Henry 78: If he had seen ‘David Harum’ first he would have made Billy Crane look like a plate of cold potatoes.
1928		T.A. Dorgan in Zwilling TAD Lex. (1993) 28: (Trying to lose an awful pest before going on with the party) Cold potato.
1939		D. Maurer ‘Prostitutes and Criminal Argots’ in Lang. Und. (1981) 116/1: cold biscuit or cold potato. An unappealing customer, or one who is difficult to arouse.
1949		R. Park Poor Man’s Orange 14: Behind her glowing russet face, Dolour looked like a cold potato.