as in the word pookaroonie being a diminutive of the word pook

I'd read this as pookeroonie is the diminutive form of Pook.

A nickname diminishes a person's social standing. It makes them lesser in relation to you.

I'm sorry you took it that way. As mentioned before, a diminutive is often used as an affectionate hypocoristic form. It increases (or augments) your standing within the social group. Though, I'm sure if you take offense to a nickname, folks here-abouts will cease using it.

Ceci n'est pas un seing.