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1.
1) A conjoined pair yelling simultaneously
2) (Colloq.) An occurance which is oft considered the apex of romantic pleasure, known in the medical parlance as a "simultaneous orgasm", in which both partners attain climax at the same moment. (From Siamese twins, referrencing the paired screaming and physical interconnectedness)
"How was your date?" Kathryn asked. "Oh!" Julie replied, "It was fabulous! We started the night with coffee, and ended it with a Siamese scream."
by Earnest June 26, 2006
 
2.
when both partners engaged in sexual activities orgasm simultaneously
My girlfriend is into all that sentimental sensitive shit, like siamese screams.
by Julie Cali July 15, 2006