The affixes ini (in) and ṭhan come in the sense of 'this is eaten by him' after the word ,
Thus श्राद्धं भुक्तमनेन = श्राद्धी or श्राद्धिकः \a person who has eaten obsequial food\. This relates to a person who eats on the day the Sraddha ceremony takes place; and not at any other time nor who eats stale food of sraddha. अद्य भुक्ते श्राद्धे श्वः श्राद्धिक इति माभूत् ॥ The word श्राद्ध originally means a certain ceremony, and is derived from the word श्रद्धा with the affix ञ 5.2.101. In a figurative sense, it here means the food taken at that ceremony.,