Chéngyǔ "chū ěr fǎn ěr" lǐ de "ěr" shì "nǐ" de yìsi.
In the idiom 出尔反尔, 尔 means "you."
The full ChinesePath dictionary entry for 尔 includes audio for every reading and example sentence, more example sentences, collocations, synonyms and links to the course units where it is taught.