Yockney Family History

Meaning ‘cuckoo's egg’ from an unattested Middle English regional development of Old English gēac ‘cuckoo’ with /jo:/ for the diphthong written ēa (as in south-western river-names of the type Yeo from ēa ‘river’) and the vowel eventually shortened in a three-syllable name in an old southern genitive case form with -n + ei ‘egg’.

For the structure, the meaning and the humour, compare Middle English cokenei literally ‘cock's egg’ one source of modern Cockney, which apparently also turns up in surnames: William Cockney 1703 in IGI (Tortworth Gloucs).