The term "hard currency" is a carry-over from the days when sound currency was freely convertible into "hard" metal, i.e., gold. It is used today to describe a currency which is sufficiently sound so that it is generally accepted internationally at face value.
HARD CURRENCY
Meaning of HARD CURRENCY in English
UNODC Money-Laundering terms English dictionary. Английский словарь терминов управления ООН по отмыванию денег, преступности и наркотикам. 2012