This is not strictly a legal word although it appears to refer to the concept of a limited partner; a partner not fully liable for his partners debts and liabilities, as general partners are to one another, but has his liability limited to the amount of his investment; hence, a limited partner.
However, as the notoriety of a partner's limited liability may be essential to maintaining that limitation, "silent" does not mean unknown or hidden. It means that the partner does not interfere in management ... remains silent!