Old English togædere "so as to be present in one place, in a group, in an accumulated mass," from to (see to) + gædere "together" (adv.), apparently a variant of the adverb geador "together," from Proto-Germanic *gaduri- "in a body," from PIE *ghedh- "to unite, join, fit" (see good, and compare gather).
In reference to single things, "so as to be unified or integrated," from c. 1300. Adjective meaning "self-assured, free of emotional difficulties" is first recorded 1966. German cognate zusammen has as second element the Old High German verbal cognate of English same (Old English also had tosamne "together").
Middle English slep, from Old English slæp "state of quiescence of voluntary and conscious functions; sleepiness, inactivity," from Proto-Germanic *slepaz, from the root of sleep (v.). Compare cognate Old Saxon slap, Old Frisian slep, Middle Dutch slæp, Dutch slaap, Old High German slaf, German Schlaf, Gothic sleps.
By c. 1200 as "a period of sleep." Personified in English from late 14c., on the model of Latin Somnus, Greek Hypnos. Figurative use for "repose of death" was in Old English; euphemistic put (a pet animal) to sleep "kill painlessly" is recorded from 1923. A similar imagery is in cemetery.
Sleep deprivation is attested from 1906. Sleep-walker "somnambulist" is attested from 1747; sleep-walking is from 1797. Sleep apnea is by 1976. To be able to do something in (one's) sleep "easily" is recorded as a hyperbolic phrase by 1953. Sleep apnea is by 1916.
Middle English slepen, from Old English slæpan "to be or fall asleep; lie or remain dormant or inactive" (class VII strong verb; past tense slep, past participle slæpen), from Proto-Germanic *slēpanan (source also of Old Saxon slapan, Old Frisian slepa, Middle Dutch slapen, Dutch slapen, Old High German slafen, German schlafen, Gothic slepan "to sleep"), from PIE *sleb- "to be weak, sleep," which perhaps is connected to root *sleg- "be slack, be languid," the source of slack (adj.). Sleep with "do the sex act with" is in Old English:
Gif hwa fæmnan beswice unbeweddode, and hire mid slæpe ... [Laws of King Alfred, c. 900]
Related: Slept; sleeping. There is no cognate form of the verb in Scandinavian. The usual PIE root is *swep-. The meaning "to rest as in the grave" is from Old English. In reference to parts of the body, "be numb through stoppage of circulation," late Old English. The sense of "provide or afford sleeping accommodations for" is by 1848, American English.
To sleep in "remain in bed in the morning" is by 1827; to sleep out "spend the night in the open" is by 1852. To sleep (something) off "remove the effects of by sleeping" is from 1760 (sleep out in the same sense is from 1550s). To sleep on some matter "postpone decision until the following day" is from 1510s, perhaps suggesting guidance in a dream. To sleep around "have casual sex with multiple partners" is attested by 1928.
1620s, of drugs, "inducing sleep," from French hypnotique (16c.) "inclined to sleep, soporific," from Late Latin hypnoticus, from Greek hypnotikos "inclined to sleep, putting to sleep, sleepy," from hypnoun "put to sleep," from hypnos "sleep" (from PIE root *swep- "to sleep"). Modern sense of "pertaining to an induced trance" first recorded in English 1843, along with hypnotize, hypnotism, hypnotist, in the works of hypnotism pioneer Dr. James Braid. Related: Hypnotical; hypnotically.
"sleep personified; the god of sleep in Roman mythology," equivalent of Greek Hypnos, son of Night and brother of Death, 1590s, from Latin somnus "sleep, drowsiness," from PIE *swep-no-, suffixed form of root *swep- "to sleep."
word-forming element meaning "sleep," from Greek hypnos "sleep," from PIE *supno-, suffixed form of root *swep- "to sleep."
"walk in one's sleep," 1821, probably a back-formation from somnambulism, from French somnambulisme, from Modern Latin somnambulus "sleepwalker," from Latin somnus "sleep" (from PIE root *swep- "to sleep") + ambulare "to walk" (see amble (v.)). Also sometimes transitive, "walk on or over in sleep." Related: Somnambulated; somnambulator; somnambulating. Alternative verb somnambulize, somnambulise is by 1832.