畳たたみの上うえの水すい練れん

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Meanings

1

useless book learning; theory without practice

idiomatic expressionjocular, humorous

Idiomatic and often jocular; literally 'swimming practice on a tatami mat', implying knowledge that cannot be applied in real situations.

いくらマニュアルを読よんでも、畳たたみの上うえの水すい練れんだよ。
No matter how much you read the manual, it's just useless book learning.
彼かれの知ち識しきは畳たたみの上うえの水すい練れんで、実じっ際さいの仕し事ごとでは役やくに立たたない。
His knowledge is all theory and no practice; it's useless in real work.