It is a symbol for UMMO, a secret society started in the late 60's. This UFO cult believed we were in contact with aliens in another galaxy with a homeworld very much like our own. The cult was revealed as a hoax in the 80's but the rumor was there was still a ring of truth to their belief.

If the idea started on the Island after the Purge, it could still be running now even though the rest of the world might have forgotten about it.

Ummo - Wikipedia, the free encyclopedia

"References to Ummoism appeared during the third season of the hit ABC drama Lost. While the references were subtle, a graphic symbol for ummo has appeared several times, including as an image burned into a the character of Juliet Burke's lower back. The possible extent of this graphic reference has yet to be explained. Theories exist on several fan websites concerning the possibility of an alien explanation to the mysteries of LOST.[1]"

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"The Naked Gun!"

Frank Drebin remembers a lost love.

Frank: It's the same old story. Boy finds girl, boy loses girl, girl finds boy, boy forgets girl, boy remembers girl, girls dies in a tragic blimp accident over the Orange Bowl on New Year's Day.

Jane: Goodyear?

Frank: No, the worst.