This old wives' tale certainly is old. It's a myth that has been circulating among breastfeeding mothers for ages. Perhaps other nursing moms have shared with you unfounded explanations that the hops in the beer is a galactogogue, meaning that it is a substance that builds milk supply. Some may have suggested that having a glass of wine will help you to relax and improve your milk letdown reflex.
Alcohol - Breast Milk Supply Research
Though it has been a common suggestion for increasing breastmilk supply, this is one of those instances where the science doesn't support the popular opinion. In fact, scientists have actually found the exact opposite to be true. Consuming alcohol limits the production of hormones that trigger milk supply and milk letdown. If you are struggling to increase your breast milk supply, drinking alcohol is actually poor advice.
Rather than falling back on this myth to boost your milk supply, stick with proven methods of building breastmilk supply instead. If you are having serious issues with your milk supply, you may be better off avoiding alcoholic beverages altogether. However, if your supply is well established, it is okay to drink alcohol and breastfeed on occasion, without needing to pump and dump your milk. So be assured you can drink - just be sure to do it responsibly.
Mennella JA, Pepino MY. Biphasic effects of moderate drinking on prolactin during lactation. Alcoholism, Clinical and Experimental Research. 2008 Nov;32(11):1899-908.
Mennella JA, Pepino MY, Teff KL. Acute alcohol consumption disrupts the hormonal milieu of lactating women. The Journal of clinical endocrinology and metabolism.2005 Apr;90(4):1979-85.