The English word 'arch' entered the language in the fourteenth century from Old French 'arche,' which descended from Vulgar Latin *arca, a derivative of classical Latin 'arcus,' meaning 'a bow' (the weapon) or 'any curved or arched shape.' The ultimate root is PIE *h₂erkʷ-, meaning 'to bend' or 'to bow.'
The semantic connection between a bow and an architectural arch is transparent: both are curved structures that derive their strength from tension distributed along a curve. The Romans, who perfected the structural arch and spread it across their empire, used the same word 'arcus' for the weapon and the building technique. This dual meaning passed into most of the Romance languages: French 'arc,' Spanish 'arco,' Italian 'arco,' and Portuguese 'arco' all mean both 'arch' and 'bow.'
In English, the two senses eventually split into separate words. 'Arch' (from Old French 'arche') retained the architectural sense, while 'arc' (borrowed later, directly from Latin 'arcus' or from French 'arc') took on the more general geometric meaning of a curve or segment of a circle. 'Archer' (from Old French 'archier,' from Vulgar Latin *arcārius) preserves the bow-weapon connection, as does 'archery.'
The compound 'arcade' has a particularly long history. It entered English from French in the eighteenth century, from Italian 'arcata' (a row of arches). An arcade was originally a covered passageway with arched openings on one side — a common feature of Italian Renaissance architecture. In the nineteenth century, such passages were roofed in glass and filled with shops, becoming the 'shopping arcades' of European cities. In the twentieth century, 'arcade' was transferred
The prefix 'arch-' (as in 'archbishop,' 'archenemy,' 'archetype') is an entirely separate word that happens to be spelled identically. It derives from Greek 'arkhi-' (ruling, chief), from 'arkhein' (to begin, to rule). The Greek and Latin words are unrelated; their convergence in English spelling is coincidental and has caused persistent confusion.
Architecturally, the true arch — where wedge-shaped voussoirs lock together under compression with a central keystone — was known to the Etruscans and perfected by the Romans, who used it on a scale unprecedented in the ancient world. Roman aqueducts, bridges, and triumphal arches spread the technology and the vocabulary across Europe. The triumphal arch as a freestanding ceremonial monument created the association between arches and military victory that persists in structures like the Arc de Triomphe in Paris and the Arch of Titus in Rome.
In English, 'arch' as an adjective (meaning playfully teasing, cunningly roguish) is yet another unrelated word. This 'arch' is a clipped form of the prefix 'arch-' (chief), used to mean 'pre-eminent' and then ironically 'quintessentially sly.' An 'arch smile' is etymologically a 'chief smile,' a smile that is the quintessence of knowing playfulness.
The word's phonological journey from Latin 'arcus' to English 'arch' involved several steps: the Latin 'c' /k/ palatalized before the 'u' was lost in Vulgar Latin, producing a sound that Old French rendered as 'ch' /tʃ/. English borrowed this French pronunciation directly, giving us the modern /ɑːɹtʃ/.