300px|thumb|Sinmara (1893) by [[Jenny Nyström]]
In Norse mythology, Sinmara is a gýgr (giantess), usually considered a consort to the fiery jötunn Surtr, the lord of Muspelheim. Sinmara is attested solely in the poem Fjölsvinnsmál, where she is mentioned alongside Surtr in one (emended) stanza, and described as keeper of the legendary weapon Lævateinn in a later passage. Assorted theories have been proposed about the etymology of her name, and her connection with other figures in Norse mythology.
Etymology
Nightmare
The etymology of the name Sinmara is obscure. However, the name has been associated with the nightmare/succubus spirit (mara) of folklore since Árni Magnússon (Magnæus)'s Poetic Edda (1787-1828). The "-mara" ending is thought cognate with mara or "night-mare". The initial sin- element is here identified as meaning "sinew" or rather "nerves", so that the total phrase comes out as "nervous (or nerve-afflicting) nightmare". This is consistent with the attestation in the poem Fjölsvinnsmál that she is hin fölva gýgr ("the pale giantess", or perhaps "ashes-coloured giantess").
Fjölsvinnsmál
Sinmara is solely attested in the Eddaic poem Fjölsvinnsmál. The poem refers to her as a pale giantess (gýgr), so she is "probably a giantess".