Green’s Dictionary of Slang

(UK drugs) for a young city person to travel to the country where they set themselves up, probably through some form of coercion, as a temporary drug dealer in a borrowed house or flat; the actual owner is forced to act as an assistant.

2010s		UK drugs 12 May 🌐 She ran drugs all over the UK [...] She would be driven out to the countryside and left to operate from someone’s house while she dealt drugs – a practice known as ‘cuckooing’.
2018-20		T. Thorne (ed.) ‘Drill Slang Glossary’ at Forensic Linguistic Databank 🌐 Cuckooing – the process whereby the cuckoo, a drug dealer, forcibly takes over the cuckooed potential assistant’s premises and forces them to sell drugs on their behalf, by violence, coercion or degrading behaviour.
2019		Guardian 18 Sept. 🌐 Emma was being ‘cuckooed’ — her home taken over by drug dealers picking a vulnerable person to use the place as a base. The young men set themselves up in her home [...] and refused to leave.