by my tunic

What does this expression mean?

“The Richest Man in Babylon” from which this excerpt is a motivational guide based on a tale from classical times. ‘By my tunic’ is simply an oath taken on the man’s attire.
It is an oath similar to these more standard expressions:
For pity’s sake…
For goodness sake…
By the stars…
On my life…
In God’s name…
For God’s sake…
On my mother’s /wife’s/ child’s/ baby’s life…
and in the given context it just asserts the strength of the speaker’s feelings.