honest to a fault; only redeeming feature is honesty
Describes a person whose sole positive trait is honesty, often implying they lack other skills or charm. Can be used self-deprecatingly or to describe someone bluntly.
He is honest to a fault and has no other redeeming qualities.
I'm just a person whose only strength is honesty, so people don't like me.
正直 simply means 'honest' or 'frank'. 正直一遍 adds the nuance that honesty is the only good point, often with a negative or self-deprecating tone.
Compound of 正直 (honesty) and 一遍 (one thing, solely). The exact historical derivation is uncertain, but the phrase is used to mean 'only honesty'.