老公公 is an informal, affectionate term for an elderly man, similar to 'grandpa' or 'old man' in English. It can also refer specifically to one's husband's father (father-in-law), though this usage is less common in modern speech. Historically, it was also a term for eunuchs in imperial China, but this sense is now archaic and rarely used outside historical contexts. The word carries a warm, familiar tone and is often used by children or in family settings.