Green’s Dictionary of Slang

(US) a nickel, a five-cent coin.

1914		Jackson & Hellyer Vocab. Criminal Sl. 57: meig [...] A nickel; a five-cent piece.
1931		Irwin Amer. Tramp and Und. Sl. 128: Meig.– A five cent piece; also one cent when found in the plural, as ‘fifty meigs’ for fifty cents.
1962		Ragen & Finston World’s Toughest Prison 808: meig – A five-cent piece; also, one cent when found in the plural, as ‘fifty meigs’.