early 15c., securite, "state or condition of being safe from danger or harm;" mid-15c., "freedom from care or anxiety" (a sense now archaic), from Old French securite and directly from Latin securitas "freedom from care," from securus "free from care" (see secure (adj.)).

This form replaced the earlier sikerte (early 15c.), which represents an earlier borrowing of the Latin word; earlier in English in the sense of "security" was sikerhede (early 13c.); sikernesse (c. 1200). Sir Thomas Browne uses securement; Francis Bacon and Mrs. Browning have secureness. Surety is a doublet, via French.

The meaning "something which secures, that which makes safe" is from 1580s. The specific legal sense of "something pledged as a guarantee of fulfillment of an obligation" is from mid-15c. (originally a guarantee of good behavior).

The meaning "safety of a state, person, etc." is by 1941. By 1965 Security, with the capital, was generic or shorthand for "security officials; a state's security department or ministry."

The legal sense of "property in bonds" is from mid-15c.; that of "document held by a creditor as evidence of debt or property and proof of right to payment" is from 1680s. Security-check (n.) is by 1945. Phrase security blanket in figurative sense is attested by 1966, in reference to the crib blanket carried by the character Linus in the popular "Peanuts" newspaper comic strip (the blanket, and the strip, from 1956).

Israeli security service, 1964, from Modern Hebrew shin + bet, names of the initial letters of sherut bitahon (kelali) "(general) security service."

national security agency of the Soviet Union from 1954 to 1991, attested from 1955 in English, initialism (acronym) of Russian Komitet gosudarstvennoy bezopasnosti "Committee for State Security."

c. 1400, "to deposit as security," from Old French gager, gagier "to guarantee, promise, pledge, swear; bet, wager; pay," from gage "security, pledge" (see gage (n.)). Related: Gaged; gaging. For the measuring sense, see gauge (v.).

"feeling of confidence and security as to oneself," 1590s, from self- + assurance.

"colleague, associate," 1510s, from collateral (adj.). Meaning "something of value given as security" is from 1832, American English, from phrase collateral security "property, etc., given to secure the performance of a contract" (1720), in which collateral (adj.) has the sense of "aiding or confirming in a secondary way."

late 14c., sauf-gard, "protection, security, defense," from Old French sauve garde "safekeeping, safeguard" (13c.), from salve, sauve (fem. of sauf; see safe (adj.)) + garde "a keeping" (see guard (n.)). Meaning "one who protects, something that offers security from danger" is recorded from late 15c.

late 13c., from Old French ostage, hostage "kindness, hospitality; residence, dwelling; rent, tribute; compensation; guarantee, pledge, bail; person given as security or hostage" (11c., Modern French ôtage), which is of uncertain origin. Either from hoste "guest" (see host (n.1)) via notion of "a lodger held by a landlord as security" [Watkins, Barnhart]; or else from Late Latin obsidanus "condition of being held as security," from obses "hostage," from ob- "before" + base of sedere "to sit," with spelling influenced by Latin hostis. [OED, Century Dictionary]. Modern political/terrorism sense is from 1970.

internal security force of Nazi Germany, 1930, German, literally "defense squadron." Better known by its initials, S.S.

1728 in legal sense, "extinguishment by absorption," originally of real estate titles, from merge (v.), on analogy of French infinitives used as nouns (see waiver). From 1889 in the business sense "extinguishment of a security for a debt by the creditor's acceptance of a higher security;" not common until c. 1926. General meaning "any act of merging" is by 1881.