A toddy is almost always a hot drink, if made cold, it would be hard to distinguish it from a sling.
Traditional recipes consist of a spirit, mixed with hot water and a sweetener of some sort. Sometimes it is garnished with a lemon twist or even a lemon wedge, and a sprinkling of nutmeg or cinnamon.
If you try to get a clear definition of the Toddy by using the Jerry Thomas 1862 Bartenders Guide, you will find it more than a little confusing. He lumps the Toddy and Sling together, appearing to indicate that both can be hot or cold, with the only difference being a Sling is server with a grating of nutmeg.
In this section he lists recipes for:
- Apple Toddy: Sugar, cider brandy, 1/2 baked apple, boiling water, garnished with grated nutmeg.
- Brandy Toddy: Sugar, brandy, water, ice. And indicates that a "Hot Brandy Toddy" would omit the ice and use boiling water.
- Whiskey Toddy: Sugar, whiskey, water, ice.
- Gin Toddy: Sugar, gin, water, ice.
- Brandy Sling: "...same ingredients as brandy toddy, except you grate a little nutmeg on top."
- Hot Whiskey Sling: Whiskey, boiling water, garnished with grated nutmeg.
- Gin Sling: "...same ingredients as gin toddy, except you grate a little nutmeg on top."
From this it sounds like all that differentiates a Toddy from a Sling is the addition of grated nutmeg, except that the Apple Toddy includes nutmeg. It is also strange that the Hot Whiskey Sling doesn't include sugar.