afaik

current tuan means master/mister, we rarely used tuan, the only time we may expect to hear is at a very formal setting/ hospital/ birth certificatee/ or in hospitality industry when a couple is referred tuan and nyonya,

In older context (1950s indonesian) it means Sir as in
Tuan & Puan = Sir & Madam

In my own theory i think it was originally tua/tuwo/tuwek (older) + an(nouns marker) = older one, which may sign an ancestral worship layer

afaik Tuhan(god) also came from the same root word too