A purchased “soap” for common birth

Meaning

Phrase.

The derisive phrase literally imagines a small bar of soap washing away common status; it denotes a purchased ennobling office. Brotteaux’s father, enriched in contracts, bought one, so the son’s nobility is acquired rather than immemorial. The edition’s expansive “a patent to wash away his common birth” preserves the cleansing joke. “Peasant’s soap” without explanation would miss the social mechanism, while “ancient noble lineage” would reverse it.