ぐうの音も出ない (guu.no.nemo.denai) describes when you are lost for words because someone has got you in an argument. In other words, it indicates a touché moment when you are too baffled even to say “touché” or any other word. Literally, this idiom means that you cannot even utter “guu”, a tiny glottal sound you’d make when you’re too baffled or shocked to say something back.
This idiom is usually used when someone gives you a flawless, logical and/or blunt criticism or argument that you cannot rebut in the least. Some people also use this word in a positive way to describe something amazing that leaves you speechless, although that deviates from the original definition.
Usage as a modifier
This idiom is often used as “ぐうの音も出ないほど(の)〜” (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 back“, where “正論” (seiron) means “a fair argument“. You may also say “ぐうの音も出ないほど美味しい” meaning “so delicious that I can’t think of any word to say.” In these cases, this idiom modifies another expression and adds some emphasis to the statement.
Usage as Internet Slang
On the Internet, “ぐうの音も出ないほど” is often abbreviated as “ぐう” (guu) and it is combined with other words. For instance, the aforementioned expression “ぐうの音も出ないほどの正論” can be abbreviated as “ぐう正論” (guu seiron). Another example is “ぐうかわ” (guu kawa), that stands for “ぐうの音も出ないほどかわいい“, meaning “so cute or hot that I can’t think of any word to say”.
Note that these abbreviated expressions are very casual and predominantly used on the internet. Thus, some people may find you a weirdo if you use them in real life rather than on the Internet.
ぐうの音も出ない can be divided into the two parts “ぐうの音も” and “出ない”. The first part means “even the sound of ‘guu'” and the latter means “not come out”. Therefore, the idiom indicates that even a glottal sound “guu” does not come out of your mouth because you’re too surprised or baffled to say a word. Note that in this idiom 音 is pronounced as “ね (ne)”, which is different from its common pronunciation “おと (oto)”
My friend told me “What the heck is taking you so long to graduate from university?” and ぐうの音も出なかった (I was lost for words)
It was so perfect a goal that I couldn’t think of any word to say.
Ex3 (On the Internet)
That female idol is really ぐうかわ (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)