Fathom out

Meaning

To ascertain something; to deduce from the facts.

Origin

A fathom is a measure of distance - the span encompassed by the outstretched arms, from fingertip to fingertip - about six feet. To fathom is to encircle with the arms. From the 14th century onward, people embracing each other were said to be fathoming.

The source of 'fathoming out' isn't entirely clear. We can be sure that it ultimately derives from that measure.

To 'fathom something out' may be merely a general reference to the measuring of the thing by using the outstretched arms. This usage dates back to at least the 16th century. For example, this piece from Richard Eden's The decades of the Newe Worlde, 1555:

Seuen men... with theyr armes streached furthe were scarsely able too fathame them [trees] aboute.

To fathom also meant 'to get to the bottom of' or 'to take soundings about'. This probably derives from the most commonly believed derivation of 'fathoming out', i.e. the measuring of the depth of water beneath a ship by use of a weight fixed to a line which was marked out in fathoms. This 'sounding out' was known both literally and figuratively by the early 17th century. For example, this literal usage, recorded in Sir William Brereton's Travels in Holland, 1634:

Fathoming the depth of the water over against Brill, we found it there where the buoys are placed to warn all seamen of the danger of that passage, that we had not above two feet more water than the ship drew.

Cyril Tourneur's play The Revengers Tragædie, 1607, makes a figurative use of 'fathoming', or, as he would have spelled it 'fadoming':

Ladies know Lucifer fell, yet still are proude!
Now sir? wert thou as secret as thou'rt subtil,
And deepely fadomd into all estates
I would embrace thee for a neere imployment,
And thou shouldst swell in money, and be able
To make lame beggers crouch to thee.

Fathom are so strongly associated with seafaring that it is hard to feel the need to look any further than the nautical measurement, especially as this was commonplace at a time when the term 'fathom it out' was coined. It could just be that 'fathoming out' is just a generic term for measuring. From this distance in time, we aren't likely ever to be 100% sure.

See also: swing the lead.