fathom

noun

1. (US, historical)A man's armspan, generally reckoned to be six feet (about 1.8 metres). Later used to measure the depth of water, but now generally replaced by the metre outside American usage.

   “[T]he ſhipmen deemed that they drew neere to ſome countrey: And ſounded, and found it twentie fathoms: and when they had gone a little further, they ſounded againe, and found it fifteene fathoms.”

   “Full fadom fiue thy Father lies, Of his bones are Corrall made: Those are pearles that were his eies, Nothing of him that doth fade, But doth suffer a Sea-change Into someting rich, & strange”

2. (US, historical)A man's armspan, generally reckoned to be six feet (about 1.8 metres). Later used to measure the depth of water, but now generally replaced by the metre outside American usage.
3. (US)A measure of distance to shore: the nearest point to shore at which the water depth is the value quoted.

   “After we'd rowed for an hour, we found ourselves stranded ten fathoms from shore.”

   “At fifty fathoms, the waters of the Southern Ocean are dark blue.”

4. (figuratively, in-plural)An unspecified depth.
5. (archaic, figuratively, obsolete)Depth of insight; mental reach or scope.

   “Another of his fathome they haue not / To leade their buſineſſe, […]”

6. (obsolete)The act of stretching out one's arms away from the sides of the torso so that they make a straight line perpendicular to the body.
7. (obsolete)Someone or something that is embraced.

   “Thy Bride, thy choice, thy vvife, / She that is novv thy fadom, […] Kneele at thy feete, obay in euerie thing, / So euerie Father is a priuate King.”

8. (figuratively, obsolete)Control, grasp.

   “So; novv knovv I vvhere I am, me thinkes already / I graſpe beſt part of the Autumnian bleſſing / In my contentious fadome, […]”

   “Yes: / you have blovvne his ſvvolne pride to that vaſtnes, / as he beleeves the earth is in his fadom, / this makes him qute forget his humble Being: […]”

verb

1. (also, figuratively, transitive)To measure the depth of (water); to take a sounding of; to sound.
2. (archaic, obsolete, transitive)To encircle (someone or something) with outstretched arms; specifically, to measure the circumference or (rare) length of something.
3. (figuratively, transitive)Often followed by out: to deeply understand (someone or something); to get to the bottom of.

   “I can’t for the life of me fathom what this means.”

   “Otamendi’s selection ahead of Vincent Kompany was difficult to fathom and, apart from Fernandinho, City’s line-up was otherwise filled with attacking players.”

4. (obsolete, transitive)To embrace (someone or something).
5. (intransitive)To measure a depth; to sound.
6. (figuratively, intransitive)To conduct an examination or inquiry; to investigate.