מה?! לא שמעתי, יש לי בננה באוזן!
The idiomatic meaning in English doesn’t work in Hebrew, just to be clear…
Why doesn't הגב need a possessive pronoun? Like: הגב שלך?
The possessive pronoun is frequently omitted, especially when dealing with body parts, because it couldn't be anyone else's back that the monkey is on. So it is understood.