The structure of this idiom is beautifully repetitive for emphasis. It combines “clumsy mouth” (笨嘴) with “awkward tongue” (拙舌). Together, they paint a very clear picture of someone whose speech-making tools—their mouth and tongue—are simply unskillful and awkward, resulting in poor communication.
In many Western cultures, being articulate and a powerful public speaker is highly praised. Eloquence is often seen as a sign of confidence and intelligence. While this is also valued in China, there's a deep-seated cultural appreciation for humility (谦虚 - qiānxū) that gives 笨嘴拙舌 a unique role. Claiming to be 笨嘴拙舌 is a common form of polite self-deprecation. Before giving a speech, accepting a gift, or trying to express deep gratitude, a person might say, “我这个人笨嘴拙舌的…” (“I'm the kind of person who is bad with words…”). This isn't necessarily a sign of low self-esteem; rather, it's a social lubricant. It lowers the audience's expectations, shows that the speaker isn't arrogant, and frames their subsequent words as sincere, even if imperfectly delivered. This contrasts with the Western concept of “selling yourself.” While an American might start a presentation by highlighting their expertise, a Chinese counterpart might start by modestly pointing out their own linguistic clumsiness. This gesture is meant to build rapport and show respect, signaling that their intentions are more important than their rhetorical skill.
This idiom is alive and well in everyday Mandarin.
Its connotation is generally neutral to slightly negative when describing a skill, but it's not a moral judgment. It's about ability, not character.