aboveboard
Honest and open, with nothing secret or sneaky happening.
Aboveboard means honest, open, and without any attempt to hide or deceive. When someone conducts business aboveboard, they're transparent about what they're doing, with nothing secret or sneaky going on. If a student asks their teacher for extra help openly instead of trying to get answers dishonestly, that's aboveboard behavior.
Today we use aboveboard to describe any situation where people act with complete honesty. A company that keeps aboveboard financial records shows exactly where their money comes from and where it goes. A friend who's aboveboard with you tells you the truth directly, even when it's uncomfortable. The opposite would be underhanded or shady, where someone hides their real intentions or actions.
When you keep things aboveboard, you build a reputation as someone people can trust completely.