I'm looking at eggnog recipes in a cookbook that was a wedding present to Carol and me 51-plus years ago, Joy of Cooking by Irma S. Rombauer and Marion Rombauer Becker There is first a recipe for a punch-glass full of the stuff on page 48 of our 1964 edition (the first edition was in 1931), and then for "Eggnog in Quantity" on page 50.
There are two versions of the "Eggnog in Quantity" recipe, the first, a "rich and extravagant version that is correspondingly good," and the second, "less powerful, less fluffy than the preceding nog, and a boon to a creamless householder." (It seems odd to think of a "creamless householder." In the case of the second version, condensed milk is substituted. But the first seven editions of this book covered the Depression and WW II years.)
The first version of "Eggnog in Quantity," among other ingredients, calls for "2 cups of dark rum, brandy, bourbon or rye." But the writer says this:
Some people like to add a little more spirit to the . . . recipes, remembering Mark Twain's observation that "too much of anything is bad, but too much whiskey is just enough."
Which reminds me of my grandmother, Hettie Louise Johnson Stokes. She loved my mother's eggnog and each Christmas remarked on how good it was. A Southern Baptist born in Dade County, GA, in 1884, Grandmother Stokes never realized that Mom's secret ingredient was dark rum. (Mom was also a Southern Baptist but from Fulton County, GA. Nita never told Hettie about the rum. Nor did anyone else.)