One of the first Chamorro words I learned as a kid; NOT because my grandma called me this, either!
But we kids used to call each other this when at play. We all spoke English, but always had some Chamorro words sprinkled in; usually stuff like this.
The word is borrowed from the Spanish.
Caduco in Spanish can mean "expired" or "lapsed," as in expired medicine, or food that has gone beyond it's labeled shelf life. It can refer to an expired license or contract.
It can even refer to things that have fallen in use, such as fashions that are out of date.
But here's where it got transformed into our Chamorro kaduko. Spanish caduco can also mean "mentally lapsed" or "deficient," as when an elderly person has failing mental capacity.
"Tu abuelo está un poco caduco," means "Your grandfather is a bit off."
And in Portuguese, caduco can actually mean "crazy." A few Portuguese settled on Guam in Spanish times, but not enough, I would think, to make a dent in Guam slang.
When applied to a woman, it becomes kaduka.
All of this is based on the Latin word cadere, which means "to fall." Think of the English musical term cadence and even cadaver - both coming from Latin cadere.
Spanish language comic strip named Caduco