'outbuilding' originates as a modern English compound of the elements 'out' and 'building'. 'out' originates from Old English, specifically the word 'ūt', where 'ūt' meant 'out, outside'. 'building' comes from 'build' (Old English 'byldan'), meaning 'to construct'.

'out' (Old English 'ūt') combined with forms of 'building' (Middle English 'bilding'/'bulding') to form the compound 'outbuilding' in later Middle English / early Modern English, eventually becoming the modern English word 'outbuilding'.

Initially it meant 'a building located outside the main building', and over time it has retained that sense and now refers specifically to a secondary or ancillary structure on the same property.