提筆忘字 [－笔－－]

Jyutping tai4 bat1 mong4 zi6
Pinyin tí bǐ wàng zì

Definitions (CC-CEDICT)
  1. to have difficulty remembering how to write Chinese characters
Definitions (Wiktionary)
  1. phrase, idiomatic
    to forget how to write Chinese characters; to suffer from character amnesia
    (syn.) 执笔忘字, 執筆忘字