My ex- girlfriend means the past girlfriend.
Its the girl with whom we were in love in the past but not now.
Normally we don't see our ex- girlfriend in our future plans and she has to be eliminated out of our life
Example of My ex-girlfriend is here

Stewart is the ex-girlfriend of Robert Pattinson
by Kaypee October 22, 2013
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Fuck you, just cause I’m a straight white male doesn’t mean I’m bad :(
My political ex girlfriend she’s “alt TikTok
by Dumbfkker September 28, 2020
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