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'arlarse' comes from the words 'arly' and 'arse',
'arly' means to be mean to someone, or sly, and 'arse' pretty much describes itself
so 'arlarse' means to be really sly towards someone, so much so that your a complete arse for being such a meany!

A very popular term used in the slums of Liverpool, many people have no clue what we mean, and shun our amazing vocabulary!
person 1: "hey can i have some of that?"
person 2: "no"
parson 1: "Oh, go on dont be an arlarse!"
by FairyBlairy March 10, 2005