Etymology: archaic English sweven ‘dream’

An obsolete word for ‘dream’ in English is sweven. Here is an example of it “in the wild,” at the beginning of Book I of Chaucer’s House of Fame (c. 1380):

God turne us every drem to goode!
For hyt is wonder, be the roode,
To my wyt, what causeth swevenes
Eyther on morwes or on evenes,
And why th’effect folweth of somme, 5
And of somme hit shal never come;
Why that is an avision
And why this a revelacion,
Why this a drem, why that a sweven,
And noght to every man lyche even. 10 (House of Fame 1.1–10)

The word comes from Old English swefn ‘sleep, dream’ and is related to two verbs, swefan ‘sleep’ (*swebaną) and its causative swebban ‘put to sleep’ (*swabjaną), which also has the meaning ‘kill’:

forþan ic hine sweorde swebban nelle
for I do not wish to kill him with a sword. (Beowulf 679)

These terms go back to the PIE root *swep- ‘sleep,’ which has some very familiar reflexes outside of Germanic:

Greek ὕπνος ‘sleep’ (cf. hypnosishypnotism; also Ὕπνος Hypnos, the personification of Sleep and brother of Θάνατος Thanatos, ‘Death’)

Sleep and His Half-brother Death by John William Waterhouse (1874)

Latin sopor (cf. soporific) and somnus (cf. insomnia, somnambulate) < *sopnos < *swép-nos (the same formation that gives us OE swefn via Proto-Germanic *swefnaz); cf. Spanish sueño

In Persian, where PIE *sw- > xw-, the noun is خواب xwāb ‘sleep, dream’; this word is loaned into Hindi/Urdu and some other languages in the same region. Internet lore—tough to confirm, however—maintains that Leonardo DiCaprio’s Inception character Dom Cobb gets his surname from this word.