'single-directional' originates from modern English as a compound of the adjective 'single' and 'directional', where 'single' meant 'one, not multiple' and 'directional' is the adjective formed from 'direction' + '-al'.

'directional' derives from 'direction', which comes from Latin 'directio' (from 'dirigere' meaning 'to guide/straighten'); 'direction' passed into Old French and Middle English before becoming English 'direction'. 'single' comes from Old English 'single' (meaning 'one, sole') and these elements combined in modern English to form the compound 'single-directional'.

Initially, components like 'direction' and 'single' referred respectively to 'a course/line' and 'one/sole'; over time the compound came to mean specifically 'having or permitting only one direction of movement or transmission'.