The term 'unchanging-rate' is a compound word formed from 'unchanging' and 'rate'. 'Unchanging' originates from the Old English 'un-' meaning 'not' and 'changian' meaning 'to change'. 'Rate' comes from the Old French 'rate', which meant 'fixed amount'.

The concept of a 'rate' has been used in various forms since the Middle Ages, evolving from the Old French 'rate' to the modern English 'rate'.

Initially, 'rate' referred to a fixed amount or price, and this meaning has largely remained the same in modern usage.