Fathom

Very, very rarely.

Hmmm. The verb is more formal than informal or more regional than common?

I think I use it from time to time. Not commonly, but more than extremely rarely. To me it feels more informal than formal.

It is so rarely used and it seems informal. I use is mostly in exclamations: I cannot fathom what he meant by that!
See the frequency chart

AlpheccaStarsSee the frequency chart here .

Of course, "fathom" has another meaning too, albeit most often in the plural, I would think.

voynichDo you use "fathom" in the meaning of "understand" in your daily conversation?

No.

CJ

I use it and hear it occasionally in my daily life,