century (100 years)

guinea (British gold coin)

to use / to employ / to make use of, to apply / to spend

century

guinea pig

a Briton, a British person, a person from the United Kingdom

United Kingdom (a country in Europe) / (loosely, chiefly attributive) Synonym of ブリテン (Buriten, “Britain”) / (loosely) Synonym of イングランド (Ingurando, “England”) (region of Great Britain)

restricted to those under 18 years of age, due to serious sexual content

to use one's head

use one thing for many purposes / work someone hard