Head case and Odd fish

Head case noun - A person judged to be legally or medically insane.

Odd fish and head case are semantically related. In some cases you can use "Odd fish" instead a noun phrase "Head case".

Odd fish noun - Someone regarded as eccentric or crazy and standing out from a group.

Head case and odd fish are semantically related. Sometimes you can use "Head case" instead a noun phrase "Odd fish".