老头(lǎo tóu) - old man, old fellow in Mandarin

老头 (lǎo tóu) is the everyday, colloquial word for an old man. It is casual rather than respectful, somewhere between "old fellow" and "old boy" in English, and it depends entirely on who is saying it.

You often hear the fuller 老头子, and a wife using it of her husband makes it affectionate, the equivalent of "my old man". A stranger using it of a passer-by is closer to dismissive.

The trap is politeness. Never address an elderly man as 老头 to his face. The respectful forms are 老先生, 大爷 or 爷爷, and 老人家 is the safe general word. Keep 老头 for describing someone in the third person, and even then watch your tone.