Why do we say "A piece of my mind (e.g. I gave him a piece of my mind)"?

Meaning:

I said exactly what was on my mind, (usually in the context of telling someone off/scolding them)

Background:

Earliest Known Reference :
Original Letters Illustrative of English History (1824) by Sir Henry Ellis, quotes an example from 1572...
"Thus am I bolde to unfolde a peece of my mynde"

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