Algebra

if x^3 is cubic,
x^2 is quadratic,
x is linear,
and 123 is constant,
then what is \sqrt{x}?

>>10

From the online etymology dictionary:

late 14c. (adj.), in a medieval philosophical sense, from L.L. radicalis "of or having roots," from L. radix (gen. radicis) "root" (see radish).

It's a square ROOT. Roots = radicals.