'on-staff' is formed from the preposition 'on' (Modern English) + the noun 'staff' (Modern English). 'on' comes from Old English 'on' meaning 'on, in, into', and 'staff' comes from Old English 'stæf' meaning 'staff, stick, rod'.

'staff' originally meant 'rod' or 'stick' in Old English ('stæf') and in Middle English the word developed additional senses (including a support or emblem). From the late Middle English period, 'staff' came to be used for a group of persons working for an employer ('the staff'), and the phrase 'on staff' developed as a simple prepositional phrase meaning 'on the staff of' or 'employed by'.

Initially, components referred to a physical 'staff' (rod) and a spatial relation 'on'; over time 'staff' acquired the sense 'group of employees', so 'on staff' evolved to mean 'being employed as a staff member'.