The anti-Semites have used the word "Zionist" instead of Jew in order to pretend that they are not anti-Semites (which itself is a way of hiding the original word Judenhass which means Jew hatred). If used as a slur Zionist just shows the ignorance of an anti-Semite. It really means the belief that Jews have the right to return to their homeland of Israel (Eretz Tzion - the Land of Zion) (called "Palestine" by the British Mandate following the Roman attempt to deny the existence of Israel).
See the definition
Zi·on (zī′ən) also Si·on (sī′ən) n.
a. The historic land of Israel as a symbol of the Jewish people.
b. The Jewish people; Israel.
A place or religious community regarded as sacredly devoted to God.
An idealized, harmonious community; utopia.
[Middle English Sion, from Old English, from Late Latin Siōn, from
Greek Seiōn, from Hebrew ṣiyyôn; see ṣwn in Semitic roots.]
American Heritage® Dictionary of the English Language, Fifth Edition.
Copyright © 2011 by Houghton Mifflin Harcourt Publishing Company.
Published by Houghton Mifflin Harcourt Publishing Company. All rights
Zion (ˈzaɪən) or Sion n
(Bible) the hill on which the city of Jerusalem stands
a. the ancient Israelites of the Bible
b. the modern Jewish nation
c. Israel as the national home of the Jewish people
(Ecclesiastical Terms) Christianity heaven regarded as the city of God and the final abode of his elect
(Sociology) any form of social organization, way of life, or life after death regarded as an ultimate goal
a. a religious community or its site, regarded as chosen by God and
under his special protection
b. an ideal theocratic community, esp any of the Christian Churches
regarded as such a community
Collins English Dictionary – Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003