Yay! Love that wicked word.)
There's fathom ['fæðəm] - a measurement of 1.8 metres or 6 feet, used when referring to the depth of water.
So when one can't fathom something, they are unable to understand it as it's too deep complicated.
It's even transcribed into Russian as 1) фатом, фадом (английская единица длины; = 6 футам, или 182 см) and is also translated as морская сажень (which is very close to the маховая сажень of 176 cm).
There's even an echo-sounding device, to measure the depth, of course :
- [fæ'ðɔmɪtə], again transliterated as фатометр.
So, 'unfathomable' has gotta be something so deep, even immeasurable, immense - ex profundis of a 'bottomless' 'abyss')).
And one of mine is surely encumber (incumber):
she was encumbered by her heavy skirts
they had arrived encumbered with families