1Currency, or a currency, in the form of banknotes, bills, etc., as opposed to coins; paper currency. Later also: unsecured paper credit as opposed to coins, banknotes, or other objects or documents viewed as having exchangeable value.
2A currency which is not convertible into other currencies, or whose price in terms of other currencies is expected to fall.
soft currency/ˌsɒf(t) ˈkʌrənsi/ /ˌsɒf(t) ˈkʌr(ə)nsi/
Mid 19th century. From soft + currency.