Opposition to any of various political unions; the policy or principles of an anti-unionist group or party.
Used originally to denote the movement opposed to the union of Great Britain and Ireland, and (later) to Northern Ireland remaining part of the United Kingdom; in the United States denoting opposition to the Union side during the American Civil War (1861–5). Compare "unionism", 2.
2Opposition to the system, principles, or practice of trade unions.
Late 18th century. Partly from anti- + union + -ism.