Originally a Hebrew word, popularized in English by the cartoonist Herb Gardner. A `sad sack,' a loser, a person who can't make any thing or any situation work right for him or her; unassertive, shy, timid. Reference: `The Joys of Yiddish,' by Leo Rosten. His definition is "An innocuous, ineffectual, weak, helpless or hapless unfortunate." There are other spellings also.
The poor little fourth-grader was such a nebbish that she peed in her pants, rather than speaking up and asking the teacher for a hall pass to go to the bathroom.