Construction I need help clarifying in Otfried Preußler's "Krabat"

Question

I was reading the novel "Krabat" and came across the following sentences:

»Ja«, sagte Lobosch, »heuer zum letzten Mal. Denn nun bin ich hier Lehrjunge auf der Mühle.« Das sagte er voller Stolz, und die Mühlknappen dachten sich ihr's dabei.«

The bit that's confusing me is that last clause, "… und die Mühlknappen dachten sich ihr's dabei." I found a website that lists all the archaisms or otherwise strange idioms in this book and it lists that clause as being equivalent to "und die Mühlknappen dachten sich ihre Antwort darauf." So my understanding is that "ihr's," a contraction of "ihr es," is here an abbreviation of "ihre Antwort darauf," but I don't understand how exactly you get from one to the other.

Answer 1

It's not ihr es, but ihrs / ihres (theirs).

I think, today you would rather say something like:

Sie dachten sich ihren Teil dabei.
^{They had their own thoughts on the matter.}

But as you see: der Teil is nothing really specific. It just stands for what comes to their mind.

A sentence like

Ich denke mir meinen Teil und schweige.

means,

I keep my thoughts to myself and don't say anything.

I could shorten that to

Ich denke mir meins und halte den Mund.

Meins is here just my something since no one knows, what I only think but don't say.