Svefnþorn
The "sleep thorn" of Old Norse saga, used to put someone into an enchanted sleep. The word is medieval; the drawn stave is not, and no two sources draw it the same way.
The drawing above shows one form of the symbol. Historical examples vary (sometimes considerably), and no single version should be treated as canonical.
Earliest attestation: Word 13th century; the drawn staves post-medieval
The oldest surviving appearance we can actually point to.
The svefnþorn appears in several Old Norse sagas: Vǫlsunga saga, where Óðinn uses one on Brynhildr, and Hrólfs saga kraka and Gǫngu-Hrólfs saga among others. The medieval texts describe an object stuck into or under someone, never a diagram.
Historical use
The visual designs now sold as svefnþorn come from post-medieval Icelandic manuscripts, and they disagree with each other: one manuscript gives four harpoon-like staves, another a quite different arrangement. There is no single historical svefnþorn design.
Modern use
Used in modern heathen practice and tattoo work, usually in the four-stave form, and often presented as Viking-Age. That last part is the error: the drawn staves come from post-medieval Icelandic manuscripts, so the design a tattoo reproduces is centuries younger than the word it is named for. How common the four-stave form now is, we can only observe.
Common misconceptions
Claims about this symbol that circulate widely and do not hold up.
A medieval word does not guarantee a medieval picture. The svefnþorn is a case where the name is medieval, the image is early modern, and the two get presented as one thing.
Sources
Full references on the sources page.