Performing a "JLS Impression" generally involves courting, flirting, admiring etc. young girls, but not so young that it becomes creepy and illegal.

This is derived from the fact that the British "band" JLS's audience tends to consist of 15/16 year old girls. Hence, if a guy more than 2 years the said-girl's senior makes a move, he is doing a JLS impression.
"Dude, did you see Carl's new girlfriend?" "No, I haven't met her yet" "Mate, she's 16!" "So?" "Carl's 18, he's doing his JLS Impression again!"

Professor Green: "I might even cover my naked body in sticky tape and run through All-Saints and come out doin' my JLS impression!"
by TKGrimm345 December 26, 2011
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