Green’s Dictionary of Slang

yawney n.

also yawny

[dial.; their mouth yawns open in stupidity]

a fool, a simpleton; also as adj., simple.

1890–1904		Farmer & Henley Sl. and Its Analogues.
1955		P. White Tree of Man (1956) 472: She was a big yawny sort of a girl, a country girl.