Thumbs Up for Etymology

We all have heard “the rule of thumb”
But don’t know rightly where it’s from.
A common myth: In days of yore,
An angry husband used it for
An upper limit on how wide
A stick he used to beat his bride.
In truth, it’s far more likely that
You’d put your thumb within a vat
To test the heat of beer you brewed.
(The olden ways were kind of crude.)
It also seems that in a pinch,
Most men have thumbs about an inch
From joint to nail tip. Anyway,
I use the phrase without dismay.