BYOB is an acronym most commonly meant to stand for "bring your own beer", "bring your own bottle", "bring your own beverage" or "bring your own booze". The term "booze" is slang for alcoholic drinks. It is also commonly used to mean "bring your own beef" at a barbecue.
BYOB is often placed on an invitation to indicate that the host will not be providing alcohol and that guests are welcome to bring their own. It is also frequently used by regular bars, restaurants, or strip clubs which do not have licenses to serve liquor or alcoholic beverages in general; an alternate term for this is brownbagging.
Several other meanings have been used, but as they are neither widely used nor recognised, they are only understood when expanded. For example, "bring your own bagels", "bring your own bhang", "bring your own Bat'leth" (said in an episode of Star Trek: Voyager) or "build your own box" in hacker jargon. Overall it is a very popular acronym that is often used to mean different things by intent.
In Australia, the colloquialism is often changed slightly to the acronym BYOG, meaning "bring your own grog". This is frequently used to describe a party where guests are expected to bring their own alcoholic beverages.
In Hong Kong, BYOB is abbreviated as Bring Your Own Bag, an environmental public service announcement campaign in the 1990s to encourage people to keep the use of plastic shopping bags to a minimum. In Singapore, this acronym is also in use, where there is a Bring Your Own Bag day on the first Wednesday of every month, to encourage the reduction of plastic bag usage at stores.
In Calgary, Canada, it is known during the days of the Calgary Exhibition and Stampede to mean "bring your own both", where "both" refers to beer and beef. Street slang in Calgary during the stampede has sometimes adapted this acronym to mean "bring your own breasts", in reference to the famous Cowboys Nightclub which pays for its waitresses' breast implants.
The term is cited by some online sources to have been used first in the early 1970s to mean "bring your own bottle", which other meanings only taking hold later. An alternate etymology is being discussed however, which is rumoured to be due for consideration by BBC television's popular series Balderdash & Piffle which challenges viewers to push back the official known dates of reference for various unusual words, for consideration by the Oxford English Dictionary. This alternative suggests that in the early 19th century, the term BYOB was used in society slang to mean "bring your own basket", with reference to group picnics. A basket would of course often include alcoholic beverages, but this is not believed to have been the primary focus of the term. This version is also supported by the Modern Drunkard Magazine.
It is generally recognised that the more modern subversion of the term was initiated by drinkers in the 1950s, but it is still contested as to on which side of the Atlantic it first occurred.