nouninformal, derogatory North American
A person who is excessively concerned with maintaining an appearance of propriety or respectability; a prudish or fastidious person.
adjectiveinformal, derogatory North American
Prudish, fastidious; overly concerned with respectability.
1920s; earliest use found in The Boston Globe. From nice + Nellie, Nelly, pet form of the female forenames Eleanor and Helen.