crucify v.

1814, "a cross," from Latin crux "cross," a word of uncertain origin. Sometimes said to be cognate with Irish cruach "heap, hill," Gaulish *krouka "summit," Old Norse hryggr "backbone," Old English hrycg "back." But de Vaan is suspicious:

The Celtic and Gm. forms are often reconstructed as *kr(e)u-k-, but we find vacillating vocalism within Gm.; also, the meanings 'backbone' and 'heap' are not necessarily connected. Even if the words in *kruk- from Latin and Italo-Celtic belong together, the root structure does not look PIE (and a root enlargement k is unknown), and might be interpreted as a non-IE substratum word borrowed into Italo-Celtic. But Latin may also just have borrowed the word from a contemporary language.

The figurative use for "a central difficulty" (1718) is older in English than the literal sense; perhaps it is from Latin crux interpretum "a point in a text that is impossible to interpret," the literal meaning of which is something like "crossroads of interpreters." But Century Dictionary ascribes it to "the cross as an instrument of torture; hence anything that puzzles or vexes in a high degree ...." Extended sense of "central point" is attested by 1888.

Proto-Indo-European root meaning "to stick, fix." 

It forms all or part of: affix; crucifix; crucify; dig; dike; ditch; fibula; fiche; fichu; fix; fixate; fixation; fixity; fixture; infibulate; infibulation; microfiche; prefix; suffix; transfix.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dehi- "wall;" Old Persian dida "wall, stronghold, fortress," Persian diz; Latin figere "to fix, fasten, drive, thrust in; pierce through, transfix;" Lithuanian dygstu, dygti "germinate;" Old Irish dingid "presses, thrusts down;" Old English dic "trench, ditch," Dutch dijk "dam."