Cheerio! A Book of Punches and Cocktails by Charles (1928, p.20)

1 slicelemon — lemon sliceacid
0.25 lemonlemon juice acid
0.67 tumblerboiling water base
1 portionIrish whiskey base
1 spoonfulsugar sweetener
  1. Fill a tumbler two-thirds full of boiling water with a spoon in the glass first, dissolve the sugar in it, and add the Irish whiskey.
  2. Squirt in the juice of a quarter lemon, garnish with a slice of lemon, and serve with a spoon.

Garnish: Lemon slice

See also: Hot Spiced Rum, Hot Irish Whiskey

Source says to put a spoon in the glass first to prevent cracking.