mid-15c., "condition of being secure," from Latin securitas, from securus "free from care" (see secure). Replacing sikerte (early 15c.), from an earlier borrowing from Latin; earlier in the sense "security" was sikerhede (early 13c.); sikernesse (c.1200).
Meaning "something which secures" is from 1580s; "safety of a state, person, etc." is from 1941. Legal sense of "property in bonds" is from mid-15c.; that of "document held by a creditor" is from 1680s. Phrase security blanket in figurative sense is attested from 1966, in reference to the crib blanket carried by the character Linus in the "Peanuts" comic strip (1956).