these words are spoken by the wedding guest to the ancient mariner. the ancient mariner hold the wedding guest with his skinny hand as he wants to share his story with the wedding guest. however the wedding guest is not interested in listening and he is also very close to the bridegroom and he wants to attend the wedding. so he tells the mariner "may 'st hear the merry din" as in all the guests have arrived and the 'merry din ' refers to the chattering voices of the guests from the wedding hall ' the wedding guest is saying this so that ancient mariner can leave him and he can attend the wedding. he clearly does not want to listen to the mariners story as he feels attending the wedding is more important.