Most likely from the intrusive R, between "yeah" (/jəː/) and a non-high vowel (/ə/, /ɪə/, /ɑː/, /ɔː/). For example, "Yeah-r-I know" (/jəɹ ʌɪ nəʊ/).
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Practice speaking and memorizing "yer" and many other words and sentences in English.