How did 'security' semantically shift to signify 'tradable financial asset'?
What semantic notions underlie the Latinate meanings of 'security' (quoting Etymonline first)
mid-15c., "condition of being secure," from Latin securitas, from securus "free from care" (see secure).
secure  Something that is secure is etymologically ‘carefree’. The word was borrowed from Latin sēcūrus, a compound adjective formed from the prefix sē- ‘without’ and cūra ‘care’ (source of English curate, cure, etc). The metaphorical extension from ‘free from care’ to ‘free from danger, safe’ took place in post-Augustan Latin. Sure is in effect a telescoped version of secure.1
to its financial meanings? I first quote OED.
e. Chiefly in plural. Originally: a document held by a creditor as a guarantee of the right to payment, or attesting ownership of property, stock, bonds, etc.; (hence) the financial asset represented by such a document. Also (orig. and chiefly U.S.): such a document issued to investors to finance a business venture.2
6. [uncountable, countable] a valuable item, such as a house, that you agree to give to somebody if you are unable to pay back the money that you have borrowed from them
- securities [plural] (finance) documents proving that somebody is the owner of shares, etc. in a particular company.
They are called securities because there is a secure financial contract that is transferable, meaning it has clear, standardized, recognized terms, so can be bought and sold via the financial markets.
Morris Pearl at Quora:
“securities” is really just a word referring to certain things that are bought and sold in the financial markets.
The origin (not really relevant to today’s usage) is that the word “security” referred to the actual paper documents. The papers had no intrinsic value, but were evidence that the owner had a secure interest in receiving something (the payments on the bonds or the dividends on the stocks, etc.)
Benjamin Schak BA Mathematics (Swarthmore), JD magna cum laude (NYU). at Quora.
The original meaning of "security," which dates back to the mid-15th century, was property pledged to guarantee some debt or promise of the owner. Starting in the 17th century, the word came to be used for a document evidencing a debt, and eventually for any document representing a financial investment. By the late 19th century, the word could refer to any tradeable investment — fixed income or equity or otherwise, and collateralized or not.
When used as a mass noun ("I pledged Blackacre and Greenacre as security on my mortgage"), "security" still has the older definition. The newer definition uses "security" as a count noun ("XYZ Company's stock is a volatile security" or "She usually holds securities for a long time").
Our modern usage doesn't quite make sense etymologically, but the same can be said of plenty of words. We don't usually sit on benches to deal with "banks," people who hold bonds don't literally cut "coupons," "preferred equity" is an obvious oxymoron, I can buy a "Eurodollar" contract in Chicago, "hedge funds" take directional views, etc.
1Word Origins (2005 2e) by John Ayto, p 445, first entry at the top on the right column.