Cream of sum yung guy is referring to, "Semen".;

It's the cream of a young guy.

The joke is used in the famous movie, "Wayne's World".
Person 1, "What do you want from Chef King?"
Person 2, " I'll Have cream of sum yung guy."
Person 1, "Haha I love that movie!"
by MEMEsRELIGIOn October 18, 2020
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