Any one adult person who engages in acts of a mentally retarded child. Someone who clearly demonstrates a lack of commitment to maturity and the responsibilities that come with it.
Dr. Doback in Step Brothers to his 40 year old son living at home who cant get a job or act like an adult:
One who acts in the manner of a jagalop. Etymology of the term is unclear but sourced from regional US American dialects. A non-gendertyped term that connotes a jagaoff who models his behavior on male gigolos.
Asking what a jagalop is such a jagalop move... Just askJames..