Weak player's diagonal
There is a Japanese phrase hebo kosumi: simply playing diagonal moves on a 'better safe than sorry' basis is the sign of a duffer.
The primary meaning may relate to the idea known to stronger players of the compromised diagonal.
The idea that the diagonal attachment should only be used when appropriate (usually attack or immediate life are the valid application) has been current here on SL as a second interpretation, consistent with the other one.
It is really a question, when White played here, whether Black had an existing stone at
(the compromised diagonal); or whether Black's existing stone was at
and Black added
when White played
(see diagonal attachment - as inferior shape).
An article in Go World explained that one aspect of Kobayashi Koichi's strength was that he didn't dismiss without thinking a 'duffer's diagonal'.