'phase-out' originates from the word 'phase,' which comes from the Greek word 'phásis,' meaning 'appearance' or 'stage.'

'phase' was adopted into English in the 17th century, and 'phase-out' emerged as a compound noun and verb in the 20th century to describe the gradual cessation of activities or products.

Initially, 'phase' referred to stages of the moon, but it evolved to describe stages in processes, leading to the modern usage of 'phase-out' as a gradual cessation.