A Grune is best defined as someone who masturbates frequently. For a Grune not to masturbate either takes lots of courage, or for the Grune to be placed in a situation where it would be socially unacceptable to do so. This is a derogatory term but can also be used in social situations:
Derogatory: Grune, you big racker; have you had a wank today?
Derogatory: Grune, put your dick away - you can wait until we're gone!
Social: Grune, we're over here - come sit with us.
Social: Has anyone seen Grune recently?