straight as an arrow - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Honest, genuine, as in You can trust Pat with the money; he's straight as an arrow. This simile alludes to the arrow's undeviating flight through the air. [Second half of 1900s]