Hi there,

Some speculation.

Ancient proverb is ["Fornkveðið er"]: "Grumpy people find fault
with everything."
"Twice man becomes child" is another saying.

I reckon that "ergist" refers to the fact: that after one has
reached one's prime, the same one's state declines.

This fact gives ground for word as grumpy, grumble, gruff manner.
Old people are often "ergileg" (in likeness of "ergi").

This state of reverse can be interpreted also as one manly opposed
to his normal state is womanly.

Or one's "ergi" [success: in Turkish] is the same one's opposite
state.

Thanks

Uoden