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).