Adamantium is in the comic book universe the hardest element in the known universe. Most recently and famously it is the virtually industructable substance that forms Wolverines skeleton and claws. It’s earliest use in the comic book universe was in the 1969 Avengers #66 as part of Ultron’s outer shell, it was also the substance that Captain America’s Shield was made out of. It has though a much longer history and as Adamantine, occurs in works such as the Aeneid, The Faerie Queene, Paradise Lost, Gulliver’s Travels, The Adventures of Tom Sawyer, The Lord of the Rings, and the film Forbidden Planet.
I think my favorite and possibly the very earliest form of this mythical Adamantine metal though was by the Greek Titan Cronos (Kronos) who “castrated his father Uranus using an adamant sickle”. The idea of Cronos (time) and the concept of castrating your own father boggles any time travelers mind.
Anyway, buy the t-shirt and you will have a wealth of mythology to discuss if anyone brings up the comic book version