honest as the day is long

(as) honest as the day is long

Very truthful and reliable. Tom is honest as the day is long—if he promised to help you move, then he'll be here. Kate would never cheat on an exam—she's as honest as the day is long.
See also: honest, long
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

honest as the day is long

OLD-FASHIONED
If someone is as honest as the day is long, they are very honest. This boy is hard-working, ambitious, and honest as the day is long.
See also: honest, long
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012
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References in periodicals archive ?
A YOUR girlfriend could be as honest as the day is long or she could well be leading you a merry dance.