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1.
adj: Used in Edinbourgeois slang, often as a comeback.
When something (especially someone or something being said) has gone too far, is over-the-top, or silly; but still in a jokey way.
When said about something that's been said it is usually followed by "from you".
Alex says "Jane, You have a massive forehead, it makes you look like a donkey."
Betty says "Mate, Plenty from you."

After seeing something silly, Charlie exclaims "That's Plenty"
by khlieeq February 21, 2008
 
2.
Plenty:

More than you might think, and less than you would presume.
Sarcastically:

"Plenty, means only one. Notify your dictionary editor".

"Plenty, is more than you think. But, more or less than that.."
by Willowdusk August 19, 2009