Middle English kēpen, from late Old English cepan (past tense cepte) "to seize, hold; seek after, desire," also "to observe or carry out in practice; look out for, regard, pay attention to," from Proto-Germanic *kopjan, which is of uncertain origin. Old English cepan was used c. 1000 to render Latin observare, so perhaps it is related to Old English capian "to look" (from Proto-Germanic *kap-), which would make the basic sense "to keep an eye on, see to it."
The word prob. belonged primarily to the vulgar and non-literary stratum of the language; but it comes up suddenly into literary use c. 1000, and that in many senses, indicating considerable previous development. [OED]
The senses exploded in Middle English: "to guard, defend" (12c.); "restrain (someone) from doing something" (early 13c.); "take care of, look after; protect or preserve (someone or something) from harm, damage, etc." (mid-13c.); "preserve, maintain, carry on" a shop, store, etc. (mid-14c.); "prevent from entering or leaving, force to remain or stay" (late 14c.); "preserve (something) without loss or change," also "not divulge" a secret, private information, etc., also "to last without spoiling" (late 14c.); "continue on" (a course, road, etc.), "adhere to" a course of action (late 14c.); "stay or remain" (early 15c.); "to continue" (doing something) (mid-15c.). It is used to translate both Latin conservare "preserve, keep safe" and tenere "to keep, retain."
From 1540s as "maintain for ready use;" 1706 as "have habitually in stock for sale." Meaning "financially support and privately control" (usually in reference to mistresses) is from 1540s; meaning "maintain in proper order" (of books, accounts) is from 1550s.
To keep at "work persistently" is from 1825; to keep on "continue, persist" is from 1580s. To keep up is from 1630s as "continue alongside, proceed in pace with," 1660s as "maintain in good order or condition, retain, preserve," 1680s as "support, hold in an existing state." To keep it up "continue (something) vigorously" is from 1752. To keep to "restrict (oneself) to" is from 1711. To keep off (trans.) "hinder from approach or attack" is from 1540s; to keep out (trans.) "prevent from entering" is from early 15c.
mid-15c., from guard (n.) or from Old French garder "to keep watch over, guard, protect, maintain, preserve" (corresponding to Old North French warder, see gu-), from Frankish *wardon, from Proto-Germanic *wardon "to guard" (from PIE root *wer- (3) "perceive, watch out for"). Italian guardare, Spanish guardar also are from Germanic. Related: Guarded; guarding.
1610s, "something stored up," from reserve (v.) or from French réserve, a back-formation from reserver "set aside, withhold," from Latin reservare "keep back, save up; retain, preserve," from re- "back" (see re-) + servare "to keep, save, preserve, protect" (from PIE root *ser- (1) "to protect").
Meaning "self-imposed restraint on freedom of words or actions; habit of keeping back the feelings" is from 1650s. The meaning "district or place set apart for some particular use" is by 1805. The sense of "amount of capital kept on hand to meet probable expenses or demand" is by 1866. That of "amount of natural resources known to exist in a particular region" is by 1912. As an adjective, "kept in reserve," by 1719.
The military sense of "body of troops withheld from action to serve as reinforcements, etc." is from 1640s; that of "national emergency defense or auxiliary military force" (reserves) is by 1866.
Another theory traces it through Medieval Latin forestis, originally "forest preserve, game preserve," from Latin forum in legal sense "court, judgment;" in other words "land subject to a ban" [Buck]. Replaced Old English wudu (see wood (n.)). Spanish and Portuguese floresta have been influenced by flor "flower."
"full of venom, noxious or hurtful by means of venom," c. 1300, from Anglo-French venimeus, Old French venimos (12c., Modern French venimeux), from venim (see venom). Earliest recorded use is figurative; literal sense by early 14c. Biologists have tended to preserve a distinction between venomous and poisonous that the general language has forgotten: venomous is applied to what bites or stings to inject toxins, poisonous to what unload toxins when eaten. Related: Venomously; venomousness.
late 14c., compote, "mixture of stewed fruits, a preserve," from Old French composte "mixture of leaves, manure, etc., for fertilizing land" (13c.), also "condiment," from Vulgar Latin *composita, noun use of fem. of Latin compositus, past participle of componere "to put together," from com "with, together" (see com-) + ponere "to place" (see position (n.)).
The fertilizer sense is attested in English from 1580s, and the French word in this sense is a 19th century borrowing from English. The condiment sense now goes with compote, a later borrowing from French.
It is the hypothetical source of/evidence for its existence is provided by: Old English Old English borgian "to lend, be surety for;" Old Church Slavonic brěgo "I preserve, guard," Lithuanian bìrginti "be parsimonious." But, absent other possible cognates, Boutkan writes that it is not certainly Indo-European and "probably a substratum word."