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Mensch is a noun in Yiddish and German.
Yiddish meaning and use
In Yiddish (from which the word migrated into American English), mensch (מענטש) roughly means "a good person." A mensch has the qualities one would hope for in a dear friend or trusted colleague. According to author and Yiddish popularist Leo Rosten,
[A] mensch is someone to admire and emulate, someone of noble character. The key to being "a real mensch" is nothing less than character, rectitude, dignity, a sense of what is right, responsible, decorous. (Rosten, Leo. 1968. The Joys of Yiddish. New York: Pocket Books. 237)
Menschlichkeit or Mentschlekhkeyt (מענטשלעכקייט) are the properties which make one a mensch.
In Modern Israeli Hebrew, the phrase Ben Adam, or "son of Adam", is used as an exact translation of Mensch. Though this usually means simply "a person" in general, it is used to mean "a nice guy" in the same way as mensch. (The term is described at the Bauhaus Archive in Berlin. It referred to a total person, similar to Maslow's self actualized person.)
The direct opposite of a Mensch is an Un-mensch (the prefix Un meaning the opposite of a mensch: an utterly cruel or evil person).
German meaning and use
In German, mensch means human. The correct spellings are Mensch (singular, meaning (non-judgmental) human or man), Menschen (for the plural and for the singular accusative) and Menschlichkeit ("humanity").