Green’s Dictionary of Slang

1. a small drink, usu. of whisky.

1829		Clonmel Herald 13 May 4/3: Come and take a dandy with me. I’ll take you to an honest house in Church-street.
1834		Tipperary Free Press 23 Aug. 3/1: Every man furnishing himself with a half pint of XX, or a dandy of Peter Smith’s best.
1864		Hotten Sl. Dict.
1868		N. O’Donoghue Knave of Clubs I 12: You're [...] ready for a swill with any one as asks you to have one. Come on to the lush-house, and have a dandy, do.
1873		Sl. Dict.
1883		H. Smart Hawkins Ch. vi n.p.: It’s beautiful punch – ah, well, as you’re so pressing, I’ll just take another dandy [F&H].
1997		Share Slanguage.

2. (also dandy glass) the glass in which it is served.

1965		(con. 1940s) B. Behan Confessions 108: The lad will have a dandy glass of stout.
1997		Share Slanguage.