This is basically an idiom that is rarely used today, but it actually means: You (or someone else) have/has made a big mistake, that this mistake is almost impossible to fix or will take a long time to; and has inevitably created a pandora's box; and/or has opened up a can of worms.

In other words, it is basically like making a huge mistake that will take almost forever to clean up. (Spilled Milk is the metaphor for the mistake.) Similar to: drop the ball
(Ex. It's easier to spill a glass of milk, but not so easy to put back all of the milk into the glass. It's easier to create a mistake, not so easy to clean up the mess of the mistake overnight. etc)

In the 1960s, civil rights activists accidentally spilled the milk on the Black (African American) community. Now, many blacks and latino's in America are in poverty and are divided into rivalry street gangs to keep each other down.
by Abraham's Adversary December 10, 2018
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When you're about about to have sex with a really hot girl but end up jizzing your pants before taking it all the way.
Guy1: Dude I almost had sex with that hot girl from the bar.

Guy2: What happened.

Guy1: I ended up spilling the milk.
by Broseiden88 March 10, 2011
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An obnoxious YouTuber who watches cartoons with his friends and makes unnecessary sexual jokes about mostly characters who are children
Person A: " Omg bro do you watch Spilling the Milk?? "

Person B: " Dude, what the fuck is your problem? "
by BrianGriffinIsMyBoyfriend November 27, 2022
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friend 1: y did u leave the vc
friend 2: imma spill milk lol
friend 1: insert smirk emoji
by jothehotcunt May 27, 2020
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when a girl's ta-tas spill out of her bra, it's spilled milk.
"dude! did you see that girl's spilled milk?!"
"there's no point in crying over spilled milk."
by got_milk December 30, 2011
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To cry over spilled milk is to remain upset about a past loss. When milk has been spilled, it is wasted and cannot be used. There is no reason to keep complaining over the loss. Note that either of the words spilled or spilt can be used.
-I can't believe we're over! You're not even going to fight for me?
-stop crying over spilled milk, we're done...you really bruised my ego.
-oh, cry me a river!
by diva1227 August 17, 2006
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