爾 here used as an adverbial suffix of classical Chinese.
爾/尔 in Classical Chinese can be used similarly to 然, roughly meaning "thus/so/in such a manner", changing the word to which it's suffixed into an adverb. Here's an example from Mencius 孟子 we can analyze:
My attempt at a faithful English translation:
A basket of rice, a platter of soup: with them one survives; without them one dies. When offered insultingly, a vagabond would not accept them; when offered trampled, even a beggar would reject them.
Here we see 爾 modify the verbs 嘑 (to insult) and 蹴 (to trample) to convert them into adverbs that indicate how the food is offered.