Arch
Etymology
From Old French "arche," from Latin "arcus" (a bow, arch, arc). The link to the archer’s bow is exact: both are curved forms under tension or compression, and the word for the weapon became the word for the building shape.
Arch meaning knowingly mischievous is a different word — from the prefix "arch-" (Greek "arkhos," chief), as in "arch-rogue" and "arch-enemy," which drifted from "principal" to "knowingly mischievous."
noun · /ARCH/
A curved structure spanning an opening that carries weight by directing it outward along the curve.
What it names
One of architecture's great inventions — the shape that let builders span wide gaps in stone and rise into vaults and domes — and gives you the eye to see the load-bearing logic in a doorway or bridge.
Example
The Romans’ round arch let them throw aqueducts clear across whole valleys.
Related idea
An arch spans an opening by curving; a buttress is the external support that absorbs the outward thrust an arch or vault pushes against. The arch creates the sideways push; the buttress resists it.
adjective · /ARCH/
Deliberately playful, knowing, or teasingly mischievous in manner.
What it names
It captures a knowing, slightly superior playfulness — an arch smile or arch remark — with no relation to the architectural curve.
Example
She gave an arch little smile, as if she knew a secret she wasn't going to share.
Related idea
Cousin to irony but lighter and warmer — an arch remark teases with a knowing wink rather than cutting like sarcasm.
verb · /ARCH/
To bend or curve something into an arch shape.
What it names
The specific motion of curving upward or outward — of the back, an eyebrow, or a bridge.
Example
The cat arched its back and hissed.
Related idea
A buttress pushes back against the outward force an arch creates; to arch is the act of forming that load-bearing curve, or of mimicking it with the body.
adjective · /ARCH/
Chief, principal, or most extreme of its kind — as in archrival or archenemy.
What it names
It elevates a noun to the top rank: not just a rival but the rival, the foremost one.
Example
Holmes finally faced his archenemy, Moriarty.
Related idea
The playful ‘arch’ manner grew out of this same ‘chief/roguish’ branch of the word.