I always thought it had to do with the compounds in soy that resemble estrogen. The idea that consuming soy (in particular unfermented soy) could act as 'feminizing'
I think it also comes from a stereotype of supposedly soulless people, who are over-invested in pop-culture and consumerism, also sedentary and in a bad physical shape generally speaking. Imagine a hardcore funko pop collector.
In that stereotype, brands like Soylent are generating a near-cult following, and consumers would buy their products for secondary reasons, like virtue signaling and other social incentives.
Obviously it is politicized, the "soyboy" is supposedly left-leaning, weak, not manly, etc...
So yeah, in conclusion, being a soyboy isn't solely the result of eating weird amounts of soy instead of a balanced diet, thus being malnourished ; but eating soy is rather seen as the telltale, most conspicuous sign of a group of symptoms, those I talked about earlier.
And, along with "cuck", research shows that soyboy is an insult mostly used by retards.
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