'spray-paint' is a compound formed from the verbs/nouns 'spray' + 'paint', created in modern English as spray-application techniques and aerosol cans became common in the 20th century.

'spray' (noun/verb) developed from earlier senses meaning a small branch or to scatter in small particles, and 'paint' comes from Old French/Latin (see below); the compound 'spray-paint' arose when spraying became a common method of applying 'paint' and was used together as a compound word.

Initially it referred simply to 'paint applied by spraying'; over time it came to refer both to the material sold in spray form (the can of paint) and the action of applying paint by spray.