One of the mysteries of the English language finally explained.
1no object To join or form a trade union; to become unionized; = "unionize". Now rare.
2with object To cause to become members of a trade union; to bring under trade union rules or organization.
Late 19th century. From trade union + -ize.
trade unionize/ˌtreɪd ˈjuːnɪənʌɪz/
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