'phase-opposed' is a compound formed from the noun 'phase' and the past-participial adjective 'opposed'. 'phase' originates from Greek, specifically the word 'phasis', where 'phasis' meant 'appearance' or 'aspect'; 'oppose' comes from Latin, specifically 'opponere', where 'ob-' meant 'against' and 'ponere' meant 'to place'.

'phase' entered English via Modern Latin/French from Greek 'phasis', while 'oppose' came from Latin 'opponere' through Old French and Middle English; the compound 'phase-opposed' arose in technical English (20th century) to describe signals with opposing phase and became common in electronics and acoustics.

Initially, 'phase' and 'oppose' had separate meanings ('appearance' and 'place against'); over time the compound came to specifically mean 'having an opposite phase' (often 180°), a technical sense used in physics, audio, and electrical engineering.