Sinmara

In Norse mythology, Sinmara is a gýgr (female jǫtunn), usually considered a consort to the fiery jǫtunn Surtr, the lord of Múspellsheimr, but wife of Mímir. Sinmara is attested solely in the poem Fjǫlsvinnsmál, 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.

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". Árni's edition also explained Sinmara to be a sort of "night fury" (Latin: furia nocturna).

Johannes Henrik Tauber Fibiger also embraced the interpretation half-way, stating the name meant "the great [night]mare", where the Sin- meaning great can be compared to Old High German sinfluth or sinvlout 'great flood'.

Adolfo Zavaroni and Emilia Reggio suggest the interpretation "Perpetual-incubus".

It has also been proposed that the sin- element may refer to sindr (Old Norse "cinders"). This is consistent with the attestation in the poem Fjǫlsvinnsmál that she is hin fǫlva gýgr "the pale giantess", or perhaps "ashes-coloured giantess". Rudolf Simek, while assessing that sin cannot be related to the term sindr, states this would equal a "meaningful interpretation in regard to the colour"; he theorizes that a more likely interpretation is "the pale (night-)mare", noting that this would fit the wife of a eldjǫtunn.

Sinew-maimer
Viktor Rydberg proposed that the name Sinmara is composed of sin, meaning "sinew", and mara, meaning  "the one that maims", noting that mara is related to the verb merja (citing Guðbrandur Vigfússon's dictionary), Rydberg concludes that the name Sinmara thus means "the one who maims by doing violence to the sinews," thus identifying her as Níðuðr's wife, who orders Völund's sinews cut to prevent his escape, in the eddic poem Vǫlundarkviða.

Fjǫlsvinnsmál
Sinmara is solely attested in the Eddaic poem Fjǫlsvinnsmál. The poem refers to her as a pale gýgr, so she is "probably a giantess".

The poem Fjǫlsvinnsmál (translated by Henry Adams Bellows) is a bridal quest, in which Svipdagr eventually gains entry to the mansion that houses his would-be bride Menglǫð. Svipdagr (under the pseudonym Vindkaldr) poses questions to the watchman Fjǫlsviðr ("Much Wise") and gathers intelligence about the mansion. He gleans the fact that the guard-hounds of the mansion can only be distracted by the meat of the cock Víðófnir. This is where Sinmara figures, as the keeper of Lævateinn, the only weapon capable of slaying the cock:

That Sinmara will only award the weapon to one who brings her the tail feather of the cock creates an insurmountable paradox to obtaining it. Fjǫlsviðr insinuates that a man may succeed in obtaining the weapon Lævateinn if a man carries a certain hard-to-obtain item to Sinmora (here she is referred to as eir aurglasis or "the goddess of gold"). Svipdagr in turn inquires what treasure it is that would so delight Sinmara (fǫlva gýgr or "the giantess pale"). Fjǫlsviðr then replies Svipdagr must bring the "bright sickle" to Sinmara, and then she will give Lævateinn to Svipdagr:

Sinmara has so far been mentioned twice explicitly, and twice by periphrases. In certain editions and translations, she is mentioned explicitly a third time as a product of emendation (in an earlier strophe than quoted above). Thus in the modified readings of certain editions and in Bellows' translation, Fjǫlsviðr names Sinmara and Surtr together, and says that the two are endangered by the rooster Víðófnir that sits atop the tree Mímameiðr:

Theories
Henry Adams Bellows comments that Sinmara is "presumably Surt's wife". In the theories of Viktor Rydberg, Sinmara is the wife of Mímir, the mother of Nótt, Bǫðvildr, "and other night díses". According to Rydberg, the byname Sinmara refers to "Mímir-Niðhad"'s "queen ordering Völund's hamstrings to be cut".

Hjalmar Falk states that "Sinmara [...] is probably no other than Hel, Loki's daughter." He says that Sinmara is specifically called hin fǫlva gýgr "the pale giantess" in Fjǫlsvinnsmál, just as the classical Roman poet Virgil speaks of the pale Orcus, a god of the underworld in Roman mythology, and that Hel is blue or half blue and half light, like the Roman goddess Proserpina, whom Saxo Grammaticus equates to Hel in his Gesta Danorum. Falk further notes that Sinmara is referred to as aurglasis Eirr, which he translates as "the goddess of the gold ring", and compares Hel's being called Gjallar sunnu gátt "wearer of the necklace" in stanza 9 of the poem Forspjallsljóð. Björn Olsen associates the kenning with veðurglasir, a name of Yggdrasill in stanza 24 of the same poem, and translates aurglasir as a name for the root system of the world-tree.