老底 is a colloquial noun that refers either to someone's personal history — especially disreputable or embarrassing secrets that can be exposed — or to a family's accumulated savings and resources. When people say 揭老底 ('expose someone's past'), they mean revealing hidden flaws or scandals; when they talk about 家底 ('family assets'), 老底 carries that same sense of inherited wealth. Context determines which meaning is at play, but both are informal and carry a hint of confidentiality or resourcefulness.