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“I don't believe you” is a phrase commonly abbreviated in Internet slang as IDBY. This sequence of four capital letters replaces the fully written phrase in modern digital communications.
Like many other chat slang abbreviations, IDBY is simply a way to say a phrase more quickly, or, more accurately, to write it more quickly. The use of these types of abbreviations mainly arose out of a need to communicate via text message. The earliest phones and mobile devices had labor-intensive text messaging, where users had to press keypad buttons multiple times to produce one letter. That is one reason many of these abbreviations started to be used commonly. Modern smartphones have touchscreen alphanumeric keypads, but it still takes a relatively long time to type on them, and so people still use these abbreviations to communicate via text message or even online from a keyboard.