Think of one small patch of east London and the people who grew up in it. The word points at that patch, and everything it carries comes along: who you are, how you sound, what you find funny. That is why the same word can name a person, a way of speaking, and a quality — they are three views of one community, not three unrelated meanings.
From the word’s history
The packet's Wiktionary entry traces it to Middle English cokenay, a spoiled child or a soft man, used in the sixteenth century by country people as a jab at city dwellers; the earlier source is uncertain, possibly cokeney, a small misshapen egg, or Cockaigne, a made-up land of luxury. The word began as an outsider's word for city people and was later narrowed to one part of London.
packet 附的維基詞典把它追到中古英語 cokenay：被寵壞的孩子、嬌弱的人。十六世紀鄉下人用它來刺城裡人。更早的來源不確定，可能是 cokeney（畸形的小蛋），也可能是 Cockaigne（傳說中的富饒之地）。也就是說，它一開始是外面的人拿來講城裡人的字，後來才縮到倫敦的一區。
Wiktionary, "cockney"
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