词穷

詞窮
cíqióng
to not know what to say; to be lost for words

CEDICT

to be unable to give a response as one is in the wrong or has given flawed arguments

Wiktionary

论辩时，因为理由或准备的材料不充分，以致于无言以对。

重編國語辭典

Entry Composition

=1 词 cí old variant of 词 cí
=1 词 cí
  1. word
  2. statement; speech; lyrics

classifier: 首
2 穷 qióng
  1. poor
  2. destitute
  3. to use up
  4. to exhaust
  5. thoroughly
  6. extremely
  7. (coll.) persistently and pointlessly

Containing this entry

理屈词穷 lǐqūcíqióng
  1. lit. having presented a flawed argument, one has nothing further to add (idiom)
  2. fig. unable to provide a convincing argument to support one's position
  3. to not have a leg to stand on
词穷理屈 cíqiónglǐqū