The original flip was made by plunging a hot fireplace poker ("flip iron" or "iron flip dog") into a concoction of rum, beer, sweetener and spices and stirring vigorously to heat and blend the drink. Adding beaten eggs to the poker-stirred potion changed the old-time flip to a yard of flannel, referring to the roughened surface caused by the cooked eggs. Today's flip always includes an egg and, depending on the version, may be served hot or cold.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.