Temple sound or a temple name
The strongest verifiable tradition states that this was originally a Tomari kata and then perfected by Itosu, which make it a part of the curricula of Shuri-derived schools such as Shotokan. Its origin is obscure with no tradition beyond Itosu. One tale has it that it is from China and an alleged temple called Jion-ji, but there is nothing to corroborate this.
The three ‘temple’ kata have been associated with each other for over a hundred years and have an obvious common origin or inspiration.