Orgeat, also known as almond syrup or almond cordial, is a type of complex syrup made from almonds and other ingredients. In the early days of bartending, fresh ingredient supply chains were unreliable. Simple basics like ice, lemons, and limes were frequently unavailable to taverns in the country, and in the city ingredients like milk, cream, and eggs would sometimes run dry. Creatively replacing ingredients was—at one time—a part of the bartender’s art. Boothby lists recipes for fake cognac and beer alongside instructions for how to clean brass. Another popular substitution was to use orgeat when cream was in short supply. Because it is typically mixed with alcohol, orgeat will keep for a very long time, and the fats from the almond work to approximate the fats from the milk, just like an almond milk from the grocery store does.
Orgeat can be made at home, but it is kind of a messy process. For most, buying premade orgeat will suffice. Small Hand Foods makes an excellent version, as does BG Reynolds. If you are feeling a bit process oriented and simultaneously inclined to squeeze a warm bag of cheesecloth filled with sugar-laden almond goop through your fingers, you can try this recipe, which worked in a pinch for us. The ultimate orgeat cocktail is the Japanese, but it works equally well in tiki classics like the Mai Tai and Fog Cutter.