Someone who loves lying in the sun to make their skin brown. Not a believer in any religion — the religion is the joke. The word compares the person stretched out on a beach towel, turning over every twenty minutes, to someone bowing before a god, because they give the sun the same faithful attention.

So the tone is light and teasing, never serious. You would use it of a friend who books beach holidays every year, or of the rows of bodies on a Mediterranean beach in August. It is gently mocking but not unkind, and most sun-worshippers would happily call themselves one.

It is more common in British writing than American, and you meet it often in travel writing, in newspaper articles about holidays, and — increasingly — in health articles warning about skin cancer, where the teasing tone turns slightly serious: 'a warning for sun-worshippers'.

Though actually — there is a second, completely literal meaning, and context keeps them apart with no trouble. In writing about history and religion, a sun-worshipper is a person who really did worship the sun as a god, as people did in ancient Egypt, in Japan, and in many other places. If the sentence is about a beach, it is the joke; if it is about a temple, it is not.

Spelling: British English usually doubles the p — worshipper. American English usually writes worshiper with one p. Both are correct in their own place.