wiktionary
ref
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/)
Russian ер(jer).
etymonline
ref
yer
representing a dialectal or vulgar pronunciation of your, attested from 1814.
