In the RN, the Buffer is the senior Rating of the Seaman branch, usually a Chief Petty Officer in big ships at least. He is responsible to the First Lieutenant (aka the Jimmy) for the state of the upper deck and all the gear thereupon - more or less the job of the bosun in days gone by.

And you CERTAINLY wouldn't want to refer to him disparagingly or he might just ping you for something you'd rather not do!

Steve