Zokushu ( 俗手 ), a Japanese go term, means "vulgar play". The Korean equivalent is pronounced soksu and is used regularly in Korean literature, whereas Japanese Go books more often relate to zokusuji.
The standard method to respond to the pincer is the 3-3 invasion at a, seeking a base where there is room for it. Other correct responses are jumping out at b or counterpincering at c or d, or tenuki. does nothing of the kind but seeks a base where there is not so much room.
After , White's base is still not sufficient and she needs to jump out to a. It is better however to omit the / exchange and jump immediately to a, because in light of future events, White may at some point wish to invade at and settle in the corner, cutting a loose. is soksu because it (or more precisely, the response it is likely to provoke from the opponent) loses this option for White and does not gain enough to be worth it.
The kind of loss in this example is conveyed in the Japanese term aji-keshi but soksu does not necessarily involve aji-keshi. It is rather about a non-standard pattern which may however occur often, a common mistake, involving a clear loss of a certain kind.