Be there or be square is an idiomatic slang expression, one uses as an ultimatum in the process of convincing a friend to attend an event.
It means either you’re coming to the party or you’ll be labeled uncool.
“Be there or be square” uses the old slang word, square which was used in the 1940’s and 50’s by jazz communities to people who were alien to new trends and who were living an exemplary but boring life.
The exact origin of the phrase “be there or be square” is debated but it started appearing in American slang during the 1950’s.
The expression was further popularized during the 1970’s and 80’s among the attendants of jazz clubs.
“Be there or be square” was first defined on Urban Dictionary in 2006.
Following 2010, memes would appear online, depicting people with rectangular faces, as well as animals with rectangular furs paired with the phrase as a caption.
“Be there or be square” memes could be found on 9GAG, iFunny as well as Reddit.