'sideline' originates from the combination of 'side' and 'line', where 'side' meant 'the edge or boundary' and 'line' referred to 'a mark or boundary'.

'sideline' evolved from the concept of marking boundaries in sports fields and later expanded to include additional activities outside one's main occupation.

Initially, it meant 'a boundary line in sports', but over time it evolved to include 'an additional job or activity'.