畳たたみ水すい練れん

noun

useless book learning; theoretical knowledge without practical ability

Idiomatic and often humorous. Literally 'swim practice on a tatami mat', implying that studying theory alone is as ineffective as trying to learn swimming on a dry floor. Used to criticize armchair experts or impractical knowledge.

• いくらマニュアルを読よんでも、畳たたみ水すい練れんでは実じっ際さいの仕し事ごとはできない。

  No matter how many manuals you read, it's just useless book learning—you can't do the actual job.

• 彼かれの知ち識しきは畳たたみ水すい練れんで、現げん場ばではまったく役やくに立たたなかった。

  His knowledge was all theory and no practice; it was completely useless on site.