Nice Nelly
/ˌnʌɪs ˈnɛli/(also Nice Nellie, nice Nelly) North American informal (depreciative )
noun
A person who is excessively concerned with maintaining an appearance of propriety or respectability; a prudish or fastidious person.
adjective
Prudish, fastidious; overly concerned with respectability.
Origin
1920s; earliest use found in The Boston Globe. From nice + Nellie, Nelly, pet-form of the female forenames Eleanor and Helen.