trade unionize
/ˌtreɪd ˈjuːnɪənʌɪz/(also trade unionise, trades unionise, trades unionize) originally British
verb
1 [no object] To join or form a trade union; to become unionized; = unionize. Now rare.
2 [with object] To cause to become members of a trade union; to bring under trade union rules or organization.
Origin
Late 19th century. From trade union + -ize.