So people called it 'tsuki-shirazu搗き知らず' (unnoticed pounding); the pronunciation of 'tsuki-shirazu' was the same as 'tsuki-shirazu着き知らず' (unnoticed arriving); people had been calling night ships 'tsuki-shirazu' because night ships often arrived at the destination while the passengers did not notice; the word 'tsuki-shirazu' reminded people of the night ships in summer, therefore, the mochi pounded unnoticed in summer began being called 'yobune' (night ship).
そこで、「搗き知らず」→「着き知らず」と言葉遊びをして、夜は暗くて船がいつ着いたのかわからないことから。