noun
useless book learning; armchair theory
Idiomatic and jocular expression for knowing theory but being unable to apply it, likened to practicing swimming in a field. The literal meaning is 'field swimming practice'.
いくらマニュアルを読んでも畑水練だよ。実際にやってみないと。
No matter how much you read the manual, it's just useless book learning. You have to actually try it.
彼の知識は畑水練で、現場では役に立たない。
His knowledge is all theory and no practice; it's useless in the field.