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1.
The use of lemon in this phrase is abbreviated from the cockney rhyming slang 'lemon tart'

lemon tart = smart
Example 1
"Don't get lemon with me son, or I'll take you outside and cut you open"

Example 2
Dumb arse gooner hater ain't so lemon now!
by apositivenuisance August 14, 2009
 
2.
"Don't get lemon" - simply means 'don't get loud' in cockney slang. Usually used in london pubs when the bloke wearing the stone island jacket gets vexed with some mug at the bar.
"Don't get lemon you gooner mug, or i'll cut your fucking jacobs off"
by _FaRSightxc2_ August 06, 2006