From Longman Dictionary of Contemporary Englishstraight arrowˌstraight ˈarrow noun [countable] American English informalHONEST someone who is very honest and moral and who never does anything exciting or unusual
Examples from the Corpus
straight arrow• Her eyesight, of course, was 20/20 straight arrow.• He was no straight arrow, this Santangelo.• Hoover was certainly, Summers shows, spectacularly indifferent to those straight arrow standards he set for his agents. 