1.
dead honest; completely straightforward; utterly sincere
An intensified form of 正直 (honest), meaning someone is honest to a fault — so honest that it can sometimes work against them. The まっ prefix adds emphasis, like 'dead' or 'completely' in English.
Usage
The prefix まっ (from 真) intensifies 正直. Often implies that the person's honesty is naive or excessive — they might say things that are better left unsaid. Can be used admiringly or with mild exasperation.
Forms
The まっ prefix is a common intensifier: 真っ白 (pure white), 真っ赤 (bright red), 真っ暗 (pitch dark), 真っ直ぐ (dead straight).