Calling someone a "Dethy" is a collective way of calling somebody a rapist, pedophile or someone with a strong yearning or desire to have sexual intercourse with a person under the age of 18.
In several places in the UK it is considered as bad or vulgar as using the word "Nigger" to describe a colored person.
It is often used in the same context as "Gay", meaning it is often used as an insult as opposed to actually describing somebody in this context.
Nice kilt, Dethy.
Stay away from me you f****** Dethy.