Does anyone know athāyaṃ word?

I think so.

  1. athāyaṃ=> atha+ayaṃ

  2. atha=> locative. singular. athāyaṃ

‘‘Yadā nu kho, āvuso, imaṃ kāyaṃ kati dhammā jahanti; athāyaṃ kāyo ujjhito avakkhitto seti, yathā kaṭṭhaṃ acetana’’nti?

  1. athāyaṃ=> atha+ayaṃ

Seems like you don’t yet know where to start the analysis of the sentence.
This is described in “Buddhavacana: A Pali Reader” by Glenn Wallis

https://books.google.com/books?id=DumN30vo7ngC&pg=PA11

Your comments are very helpful.

Unfortunately, because I’m not familiar with the English language
In order to read English
I read English using “Google Translator” & "English Dictionary"
English dictionary which I use this http://dic.daum.net/search.do?q=.&dic=eng and http://endic.naver.com/enkrEntry.nhn?sLn=kr&entryId=95701a153a80405eb9737a5be1321f34&query=.

I think right this way.

  1. athāyaṃ=> atha+ayaṃ