"Huld" is a sorceress named in the 13th C. Ynglingr Saga. In the later 13th C. Sturlungr Saga she is mentioned as a "great troll woman". "Hulda" is an Icelandic name for elves, "the hidden ones", and occurs frequently in place names and is a peculiarity of Icelandic culture. In the rest of Scandinavia Hulda are different creatures of folklore distinct from elves. The name of the sorceress or troll-woman may though be derived from "huld" flesh/fat, hylda "get fat/flesh". The Swedish "Hulda" which became popular in the 19th C is most likely related to the Common-Germanic feminine name element Huldi (West Norse Hylli, with ll pronounced as in Welsh with a merged t and l), which describes the reciprocal obligations of a superior lord and his inferior lords or followers, but also between a husband and wife. It is more distantly cognate with Gothic hulþs, “gracious, merciful”, Danish/Swedish huld (North-West Germanic hold Southern Germanic holt, West Norse hollr, which occurs as a masculine name element). It is probably a participle of a lost verb meaning "be inclined", which is why the dental is dissimilar in Gothic and the other North Germanic languages, which would otherwise be the same. Thus it is also related to Heald/Hald (bent, inclined) with some influence on the senses of the homonymous heald/hald/hold (hold, rule, protect, serve, uphold, support). Hulda is a titular character (along with Hellfried) in a late 18th C German romantic horror novel and in the 19th C. Play Halte-Hulda (Lame Hulda) by Norwegian writer Bjørnstjerne Bjørnson (although a proponent of what would become Nynorsk he wrote in the Norwegian variant of Danish that preceded Riksmål and Bokmål as the standard literary language of Norway under Danish rule) and the basis of an opera by César Franck. And earlier by Danish translations of Grimms' Hausmarchen, which rendered the German Frau Holle as Mor (Mother) Hulda, probably on the assumption that the Holle of German folklore is related to Hulda "hidden" of Scandinavian folklore. It appears many editions follow the Danish version with Mother Hulda, rather then the German Frau Holle, which may have helped its popularity in the 19th C.
This name might also have been derived from Old Norse "hell" (to conceal, to cover). There are sources that claim that Mother Hulda (a figure from a German fairy tale collected by the Grimm Brothers) is actually [based on] Hel, the goddess of the underworld in Norse mythology.