Origins
The verb 'cross' is built on one of the most culturally loaded nouns in Western civilization. Yet beneath the Christian symbolism lies a simpler physical reality: two lines meeting, a stake driven into the ground, a path traversed from one side to the other. The etymology of 'cross' traces a path from Roman execution technology through Celtic Christianity and Norse contact to become one of English's most versatile verbs.
The noun 'cross' entered Old English as 'cros,' borrowed primarily from Old Norse 'kross,' which itself came from Old Irish 'cros.' The Irish word was borrowed from Latin 'crux' (genitive 'crucis'), meaning 'stake, cross, gallows, instrument of torture.' The borrowing path — Latin to Irish to Norse to English — reflects the history of Christianity in the British Isles: Irish missionaries brought the Latin term into Celtic languages; Norse settlers in Ireland encountered it there; and the Norse carried it to England. Old English also had the native word 'rōd' (rood) for the cross of Christ, which survives in 'rood screen' and place-names like Holyrood, but 'cross' eventually dominated.
The verb 'crossen' was formed in Middle English from the noun, initially meaning 'to make the sign of the cross' — a gesture of blessing or protection. By extension it came to mean 'to mark with a cross,' and from this developed the spatial sense: 'to cross' a line, a boundary, a river meant originally to traverse it, to go from one side to the other, perhaps with an implied crossing of oneself for protection before a dangerous passage.
Proto-Indo-European Roots
Latin 'crux' is of uncertain ultimate origin. One proposal connects it to PIE *sker- (to turn, bend), suggesting the cross-piece was originally a bent or forked branch. Another sees it as a loanword from a non-Indo-European language. Whatever its deepest origin, 'crux' was productive in Latin: 'cruciāre' (to torture, to crucify — source of 'excruciating'), 'cruciālis' (of or relating to a cross), 'crucibulum' (a lamp or hanging vessel, later 'crucible' — a vessel exposed to fire, as if on a cross of flames).
The English derivative 'crucial' has a particularly interesting history. Francis Bacon, in his 'Novum Organum' (1620), used the phrase 'instantia crucis' — 'instance of the cross' — for a decisive experiment that resolves between two competing hypotheses. His metaphor was the crossroads: a point where two paths diverge and the traveler (or scientist) must choose one. Isaac Newton adopted the term as 'experimentum crucis' for his prism experiment proving that white light contains all colors. From this scientific usage, 'crucial' entered common English meaning 'decisively important,' though most speakers today have no idea they are invoking crossroads when they say something is crucial.
The spatial sense of 'cross' — to traverse from one side to the other — has generated an enormous body of compound words and idioms. 'Crossroads' (the point where two roads intersect) has been a metaphor for decision-making since antiquity; in folklore, crossroads are places of supernatural power where one might meet the devil. 'Cross-examine' (originally to question across the grain of a witness's testimony, testing it from unexpected angles) has been legal terminology since the seventeenth century. 'Cross-reference' treats knowledge as a space to be traversed in multiple directions.
Literary History
The emotional sense of 'cross' — meaning annoyed, angry, ill-tempered — appeared in the sixteenth century. The connection to the physical cross may lie in the idea of something going athwart one's wishes, cutting across one's intentions: to be 'crossed' is to be thwarted, and the state of being thwarted is to be 'cross.' Shakespeare used 'cross' freely in both senses: the star-crossed lovers of Romeo and Juliet are lovers whose destinies have been cut across by malign fate.
The word 'across' is simply 'a-' (on) + 'cross,' meaning 'on or in the form of a cross, crosswise,' and hence 'from one side to the other.' The preposition 'across' has so thoroughly absorbed this meaning that speakers rarely perceive the 'cross' within it.
In biology, 'to cross' means to hybridize — to breed two different varieties or species. Gregor Mendel's foundational experiments in genetics involved crossing pea plants, and the term 'cross' remains standard in genetics for any hybridization event. The metaphor treats the combination of two genetic lineages as an intersection of paths.
Latin Roots
The phrase 'to cross the Rubicon' — meaning to take an irreversible step — refers to Julius Caesar's crossing of the Rubicon River in 49 BCE, an act that constituted a declaration of war against the Roman Senate. The phrase has entered every European language as a metaphor for the point of no return, and it perfectly encapsulates the verb's core meaning: a boundary exists, and to cross it is to change one's situation irrevocably.
'Double-cross,' meaning to betray someone by deceiving them after pretending to cooperate, appeared in the nineteenth century, originally in the language of boxing and organized crime. The 'double' refers to crossing twice — first crossing the opponent and then crossing the supposed ally, or marking a secret second cross against the first. The exact origin is debated, but the essential image is of treacherous intersection: loyalty running in one direction is cut across by betrayal running in another.