Green’s Dictionary of Slang

1. (US) naive, worthless, unsophisticated.

1897		E.W. Townsend Chimmie Fadden 45: Dat’s de way de mug comes t’ be elected from de jay district, for he ain’t no jay himself.
1906		S. Ford Shorty McCabe 106: I’ve let myself in for some jay stunts in my time; but this [...] was the limit.
1915		S. Ford Shorty McCabe on the Job 38: It sure is a jay burg, but a lively one. [Ibid.] 167: These really was jay relations of the Twombley-Cranes.
1918		‘Max Brand’ ‘Above the Law’ in Coll. Stories (1994) 18: Two dollars for ham and – in a jay dump.
1921		N. Putnam West Broadway 15: ‘What do I want with going out to a jay place like [Hollywood] [...] I want to stay right here in civilized little old N.Y.’.

2. gullible.

1901		J. Flynt World of Graft 42: The citizens are the jayest push o’ yaps in this country.
1916		H.L. Wilson Somewhere in Red Gap 348: He puts it up with the press agent of this big hotel to have the poor things sleep up on the roof [...] so them jay New York newspapers would fall for it and print articles about these hardy sons of the forest.