"Shibboleth" is a word that can be pronounced in a whole bunch of different ways, and has no meaning; it's a pile of letters. The original use was that it was written on a piece of paper, and walked around by soldiers, asking people to pronounce the word, in order to root out outsiders and spies. Spies could learn to hide an accent by effectively re-learning the same language, but a word with no fixed pronunciation is much harder to fake. This use of it has made the word Shibboleth self-referential: A "shibboleth" is a word whose use identifies someone as belonging to a faction.

The pronunciation of it is impossible to codify, as that's the whole point. It is any one of "see-bo-let", "see-bo-leth", "shee-bo-let", "shee-bo-leth", "Seb-oh-let", "seb-oh-leth", or really any combination of those three sounds. It could even be "Syb-bowl-ith", with more worldwide accents!