老狗 literally means 'old dog', but in modern Chinese it is almost exclusively a derogatory insult — roughly equivalent to 'old bastard' or 'old son of a bitch' in English. It is used to curse an older man you despise. The literal 'old dog' sense (e.g., an aged pet) is extremely rare in everyday speech; if you mean an old canine, you would say 老狗 (lǎo gǒu) but that's not the common use. This word is very strong and vulgar — avoid using it in polite conversation.