Well, this peculiar acronym expression is very much a United Kingdom expression. It stands for "P*ssing myself laughing". It is the same as "rofl" or "lmao" in American English. The pmsl expression, like many other Internet expressions, is part of online conversation culture.
Reminder: 90% of the time, these expressions are typed in all lowercase letters. On selective occasions, you are welcome to use them in all capitals to express enthusiasm. Just remember not to type entire sentences in all caps, lest you be considered rude.
Related: 322 other common Internet expressions explained.
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