'union-supporting' is a compound of 'union' + 'supporting'. 'union' originates from Latin 'unio' via Old French 'union', where Latin 'unus' meant 'one'. 'supporting' is the present participle of 'support', which originates from Old French 'supporter' (from Late Latin 'supportare'), where 'sub-' meant 'under' and 'portare' meant 'to carry.'

'union' entered English from Old French 'union' (from Latin 'unio'). 'support' came into English via Old French 'supporter'/'soutenir' and Middle English forms such as 'supporten', later yielding modern English 'support' and the participle 'supporting'. The compound 'union-supporting' formed in modern English by combining these elements to describe a stance toward unions.

Originally, 'union' referred to 'oneness' or the act of joining, and 'support' meant 'to carry from below' or 'to assist.' Over time the compound came to mean 'favoring organized labor' rather than a literal act of carrying; the modern sense focuses on political or social support for labor unions.