たたみ うえ水練 すいれん
tatamino ueno suiren

Noun
Idiomatic Expression

1. useless book learning; knowing the theory but being unable to put it into practice; practising swimming on a tatami mat — literal

Jocular
コンピューターのマニュアルを むだけでは所詮 しょせん たたみ うえ水練 すいれんだ。
kompyūtāno manyuaruo yomudakedewa shosen tatamino ueno suirenda.
Just reading a computer manual is like practicing swimming on dry land.