A flip is a class of mixed drinks, built with spirit, sugar, egg, and spices. They can be thought of as like egg nog without the cream.
According to the Oxford English Dictionary, the term was first used in 1695 to describe a mixture of beer, rum, and sugar, heated with a red-hot iron . The iron caused the drink to froth, and this frothing engendered the name. Over time, eggs were added and the proportion of sugar increased, the beer was eliminated, and the drink ceased to be served hot.
The first bar guide to feature a flip was Jerry Thomas's 1862 How to Mix Drinks; or, The Bon-Vivant's Companion. In this work, Thomas declares that, "The essential in flips of all sorts is to produce the smoothness by repeated pouring back and forward between two vessels and beating up the eggs well in the first instance the sweetening and spices according to taste."
With time, the distinction between egg nog (a spirit, egg, cream, sugar, and spice) and a flip (a spirit, egg, sugar, spice, but no cream) was gradually codified in U.S. bar guides. In recent decades, bar guides have begun to indicate the presence of cream in a flip as optional.