Either you are being ignorant or disingenuous. I’ll assume ignorant. “Soy” as a derogatory term has both racist and misogynistic history attached to it. Soy products have long been used by Asian populations as a milk and protein substitute and was, erroneously, believed to cause increased levels of estrogen in men. Thus the insult “soy boy” was meant to denote a man who was weak, effeminate, and like unto an Asian woman.

But that’s probably not the context in which you, a likely white male 20 something, learned it.