'flasher' originates from English, formed by adding the agentive suffix '-er' to the verb 'flash', literally meaning 'one who flashes'.

'flash' is attested in Middle English (e.g. 'flasshen'), an imitative verb for a sudden burst of light or movement; the agent noun 'flasher' developed in Modern English by adding '-er' to 'flash'.

Initially, 'flash' referred to a sudden burst of light; over time the sense of a sudden display extended metaphorically to human or mechanical acts of sudden exposure or display, giving 'flasher' both the device sense and the indecent-exposure sense.