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.
No matter how many manuals you read, it's just useless book learning—you can't do the actual job.
His knowledge was all theory and no practice; it was completely useless on site.