The only funds you can contribute into your Solo 401k must come from the business adopting your Solo 401k plan.
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You probably earned Social Security from a job and business where you were not self-employed. That means the money you’re receiving from Social Security is probably not income earned by the business adopting your Solo 401k.

Because Social Security income comes from another business that isn’t the adopting employer for your Solo 401k, those funds cannot be contributed.