I always thought that meant Port Outbound, Starboard Homebound.
...on ships from the UK to India
. That would keep the POSH cabins COOL on the shady side of the ship.
ORIGIN early 20th cent.: perhaps from slang posh, denoting a dandy. There is no evidence to support the folk etymology that posh is formed from the initials of port out starboard home (referring to the practice of using the more comfortable accommodations, out of the heat of the sun, on ships between England