Huh??? You were never being particularly nice to begin with. :P
Anytime am irritably-overbearing a**h**e snorts. "No more "Mr. Nice Guy", I get really nervous --- he was already being mean and hostile up to that point, and so if THAT was his idea of GOOD behavior, then Heaven help us when he "pulls out all the stops" and acts in ways that he himself views as NOT nice...!
by QuacksO March 19, 2019
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A song by Alice Cooper.

And a phrase too if you want. self-explanatory...

Pretty much my new life starting today!
Fuck everyone else. I will never help another person ever again!

Also, this phrase is usually triggered by "Hooray for me and Fuck you!"
Fuck this, Fuck everybody! It's always Hooray for me and fuck you. Well thats it, No More Mr. Nice Guy!

you know what.. i'm gonna listen to that song right now!

I used to be such a sweet, sweet thing
'Til they got a hold of me.
I opened doors for little old ladies,
I helped the blind to see.
I got no friends 'cause they read the papers.
They can't be seen, with me and I'm gettin' real shot down
And I'm feeling mean.

No more Mister Nice Guy
No more Mister Clean
No more Mister Nice Guy
They say, he's sick he's obscene

I got no friends 'cause they read the papers.
They can't be seen, with me and I'm gettin' real shot down
And I'm, I'm gettin' mean.

No more Mister Nice Guy
No more Mister Clean
No more Mister Nice Guy
They say, he's sick he's obscene

My dog bit me on the leg today
My cat clawwed my eyes
Mom's been thrown out the social circle
And dad has to hide
I went to church, incognito
When everybody rose, the Reverand Smith,
He recognized me,
And punched me in the nose
He said,

No more Mister Nice Guy
No more Mister Clean
No more Mister Nice Guy
He said, you're sick, you're obscene

No more Mister Nice Guy
No more Mister Clean
No more Mister Nice Guy
He said, you're sick, you're obscene
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