'plugin' originates from English, specifically a compound of the words 'plug' and 'in', where 'plug' originally meant a stopper or something that fills an opening and 'in' denotes insertion.

'plug' appears in Middle English (e.g. 'plugge') and 'in' from Old English 'in'; the compound 'plug in' was used for physical insertion and later, in late 20th-century computing, the single-word noun 'plugin' was coined to describe detachable hardware modules and then software extensions.

Initially, 'plug' referred to a physical stopper and 'plug in' meant to insert physically; over time the compound evolved so that 'plugin' came to mean detachable hardware units and, later, software modules that extend a program's functionality.