Below is a list of words and phrases that have caught my attention in recent years. Most are out of fashion, or were never much in fashion over the past 50-100 years. Some are just interesting. It is likely that many are not English (I plan for this to be a living document, updating it as I find new words that catch my eye). New items awaiting a call-out in a standard post are marked with a “+”.
A-E
aglæcwif – (noun, Anglo-Saxon, disputed) a female warrior; alt: “monster”; applied to Grendel’s mother “A wretch of a woman, vile crone”
caol áit – (noun) thin place (lit, unconfirmed by standard sources, but prevalent online).
+ carr – (noun) a bog or marsh; marshy ground, swampland
chimæra – (noun) a foolish, incongruous, or vain thought or product of the imagination; dream (multiple definitions at link)
+ díchelt – (adjective) Old Irish, concealed
drycræft – (noun) magic; magical art; sorcery
dweomer/dwimmer – (noun) magic, magic arts; sorcery; spell; occult art.
entre chien et loup – (“adverb”, French idiom) between a dog and a wolf (lit); at dusk, at twilight, gloaming
F-J
+ fáith – (noun) Old Irish, seer, soothsayer
floccinaucinihilipilification – (noun) the act or habit of describing or regarding something as unimportant, of having no value or being worthless. [Ed.: Much like the word itself.]
+ fylgja – (noun) kind of guardian angel, guardian spirit, tutelary entity, which was held to follow each person or family and the relationship being affixed or bound at the process or ceremony of naming; alternately: a Norse name for the ‘spirit’/fetch that can shape-shift into an animal while a sorcerer is asleep ‘dreaming’ [unverified]
gift – (noun, German euphemism) poison; toxin; venom
gloaming – (noun) twilight, as at early morning (dawn) or (especially) early evening; dusk
+ idir eatarthu – (preposition/noun) Old Irish, “Betwixt and between”
K-O
kismet – (noun) fate; a predetermined or unavoidable destiny.
lugubrious – (adjective) gloomy, mournful or dismal, especially to an exaggerated degree
P-T
phantasmagoria – (noun) popular 18th- and 19th-century form of theatre entertainment whereby ghostly apparitions are formed; a dreamlike state where real and imagined elements are blurred together
scáthach – (adjective, proper noun if capitalized, Irish) shady, shadowy
sceadugenga – (noun) one who walks in darkness
shadowplay – ancient form of storytelling and entertainment using opaque, often articulated figures in front of an illuminated backdrop to create the illusion of moving images.
slúagh – (noun, Middle Irish) army, host; alt: sluagh na marbh (‘host of the dead’), were the hosts of the unforgiven dead in Scottish Gaelic folklore; associated with the Wild Hunt.
sweven – (noun) a dream, vision
tristesse – (noun, French) sadness
U-Z
vagary – (noun) an erratic, unpredictable occurrence or action; an impulsive or illogical desire; a caprice or whim.
warg – (noun) a type of particularly wild or hostile wolf (Tolkien revival, from Old Norse: vargr). To enter the body of an animal (usage started with GRRM).
wergeld – (noun) blood money, the monetary value assigned to a person, set according to their rank, used to determine the compensation paid by the perpetrator of a crime to the victim in the case of injury or to the victim’s kindred in the case of homicide. Alternatives: wehrgeld, weregeld, weregild, wergild
wight – (noun) a living creature, especially a human being; a ghost, deity or other supernatural entity.
sceadugenga 
gloaming – I like it!
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It’s been one of my favorite words for about a year, maybe closer to two.
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