勿信 is a formal, concise imperative phrase meaning 'do not believe' or 'do not trust'. It combines 勿 (wù), an archaic or literary negative command particle meaning 'do not', and 信 (xìn), meaning 'to believe' or 'to trust'. Unlike the more common modern negative construction 不要相信 (bú yào xiāng xìn), 勿信 is highly condensed and carries a tone of authority, warning, or official instruction — often seen in public notices, safety warnings, or classical-style admonitions.

This phrase is rarely used in casual speech or conversation; instead, it appears in written contexts such as cautionary signs ('勿信陌生来电' — 'Do not believe unsolicited calls'), internet safety tips, or satirical or rhetorical writing imitating classical Chinese brevity. Its gravity and formality make it unsuitable for polite requests among peers but effective for urgent, no-nonsense advice.