How was POSH popularised in Chitty Bang?
Personally, I think the WOM (word of mouth) of old gits like my Dad stand to greater reason, so I'm sticking with it originating from Port Out Starboard Home. It matters not that it wasn't stamped on tickets, although granted that might have provided some evidence.
This is what GPT said:
The origin of the term "posh" is not definitively traced to "Port Out Starboard Home." While this explanation is often cited, there isn't strong evidence to support it. "Port Out Starboard Home" refers to the preferred cabins on ships traveling between England and India during the British Raj, where passengers would have the cooler side of the ship.
The word "posh" itself is believed to have originated in the 19th century as a slang term among the upper class in Britain, meaning elegant, stylish, or luxurious. The connection to "Port Out Starboard Home" may have been a later invention or a coincidental backronym.
Language etymology can be complex and sometimes elusive, and the true origin of certain words and phrases may remain uncertain. In the case of "posh," while the connection to ship cabins is widely known, its exact origins are still a matter of speculation.