ぐうの音も出ない (ぐうのねもでない, guu no ne mo denai) is an advanced Japanese idiom that means “be lost for words (because someone has made a really good point in an argument against you)”. In other words, it indicates a touché moment that leaves you speechless and unable to say even “guu”: a tiny glottal sound people make in their throat when they try to say something back but cannot think of any word.
This expression is usually used when someone gives you a flawless, logical and/or blunt criticism or argument that you cannot deny or rebut at all. Note that some people also use this expression to describe something exceptional that makes you speechless, although this usage deviates from the original definition.
This idiom is often used as itself, or as a phrase “ぐうの音も出ないほど…” (guu no ne mo denai hodo …), meaning “so … that I can’t think of any word to say”. For instance, “ぐうの音も出ないほどの正論” means “so fair an argument that I can’t think of any word to say to deny it”, where “正論” means “a fair argument”. Some young people also use this expression to describe their excitement, as in “ぐうの音も出ないほど美味しい”, meaning “so tasty that I can’t think of any word to say.” In this case, ぐうの音も出ないほど modifies and intensifies the adjective 美味しい (tasty/delicious).
As Internet (Weird) Slang
On the Internet, “ぐうの音も出ないほど” is often abbreviated as “ぐう” (guu) and combined with other words. For instance, the expression “ぐうの音も出ないほどの正論” is abbreviated as “ぐう正論” (guu seiron) on the internet. Another example is “ぐうかわ” (guu kawa), which stands for “ぐうの音も出ないほどかわいい” and means “so cute/hot that I can’t think of any word to say”.
Note that these abbreviated expressions sound very casual/slang, and they are used only by a limited number of people on the internet. Therefore, some people will find you weird if you use them in real life.
My friend told me “Why the heck it is taking so long for you to graduate from university?” and I was lost for words.
It was so perfect a goal that I couldn’t think of any word to say.
Ex3 (Internet Slang)
That female idol is really so hot that I can’t think of any word to say.
Synonyms: 返答に詰まる (be stumped; unable to answer)
Synonyms in English: be stumped, lost for words, touché
Related words and phrases: 正論 (fair argument), 音 (sound), 出る (come out), かわいい (hot)