Mapainik is a colloquial term for a member or supporter of Mapai

Labor (Israel)

The Israeli Labor Party , commonly known as HaAvoda , is a social-democratic and labour Zionist political party in Israel. The party is an observer member of both Socialist International and the Party of European Socialists. The Israeli Labor Party was established in 1968 by a merger of Mapai,...

, the historical Israeli Labour Party. The party has since changed its name to Ha'Avoda, but one can occasionally still encounter the term as an informal description for "old school" former and present members of Labour in Israel, especially those who defend a secular form of Zionism

Zionism

Zionism is a Jewish political movement that, in its broadest sense, has supported the self-determination of the Jewish people in a sovereign Jewish national homeland. Since the establishment of the State of Israel, the Zionist movement continues primarily to advocate on behalf of the Jewish state...

 against more religious interpretations of the same ideology.

See also: -nik

-nik

The English suffix -nik is of Slavic origin. It approximately corresponds to the suffix "-er" and nearly always denotes an agent noun...