The word "bar" refers to the barrier or countertop over which customers are served.
A Facebook page called "Daddy and Mummy Quotes" shared an image on December 23, 2018 that said "How old were you when you learned that "BAR" stood for beer and alcohol room?" The post has garnered almost 75k views in the last 24 hours.
"Bar" in common usage describes an "establishment that sells and serves alcoholic drinks and often offers other forms of entertainment such as music and shows."
The word "bar" comes from the 12th-century Old French word "barre", meaning a stake of metal used to fasten a door. Over the next few centuries, it came to refer to both the piece of metal itself and any sort of obstruction or barrier. The word "bar" to describe a drinking establishment comes from the late 16th century. The word was used to refer to the countertop between customers and staff, as it was a barrier.
According to Renegade Brewing, "the idea that bar stands for beer and alcohol room is what is referred to as false etymology. A false etymology is a falsely held belief about the origin and meaning of a word."
[Updated 11/11/2021 with the correct etymology.]