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Brackets that represent an unfilled tick-box. Hence the writer is asserting that the statement following the brackets is untrue.

Often used for humorous effect, and to contradict or point out an error in what someone has just said. Somewhat similar in meaning to adding "NOT" after a statement. Multiple tick-boxes are often strung together, like a check-list of true and false statements.

{ } is the opposite of {x}, a checked tick-box which means the statement is true.

(Note - curly brackets are used in the examples, since urbandictionary uses square brackets to denote links. However in real usage, square brackets should always be used.)

the use of square brackets surrounding any word or phrase indicates an understood expletive before the bracketed word or phrase and emphasizes strong feelings on a subject. The reader then has the option of inserting the expletive at their discretion.

For instance, work could be read as "fuck work."

Note: Brackets can also be used when speaking by using hand gestures. While using the word you wish to bracket, simply put your hands up with your thumb and middle finger forming the shape of the square brackets.

in UrbDic you use these: to highlight a word.

when you slam on the keyboard while you are raging and you get these random characters and you dont even know how you got them on the screen.