It's not just Line of Duty acronyms that can leave us scratching our heads on a Sunday night – BBC One's hit police corruption drama is also keen on dishing out all manner of cop jargon and doing its audience the credit of asking them to keep up.
From yellow notices to production orders, Line of Duty loves its police patter – and tonight's episode (the third of series six) offered up another example, with the use of the term "cuckooing".
AC-12 refer to "cuckooing" throughout the episode in regards to their latest operation and poor unfortunate Terry Boyle (Tommy Jessop) – but what does it actually mean?
Well, "cuckooing" is a term invented by the police to refer to a crime in which drug dealers take over the home of a vulnerable person - in this case, Terry - to use it as a base.
From their new HQ, the criminals take part in "county lines" drug trafficking, i.e. trafficking drugs into rural areas and smaller towns, away from major cities, with vulnerable children often being recruited to act as dealers. (The term "country lines" refers to the phone numbers - or "lines" - dedicated to this activity.)
So why "cuckooing"? Well, the term takes inspiration from a cuckoo's habit of taking over other birds' nests to house its young – just as criminals took over Terry's flat.
Its use is thought to have originated in the early 1990s, with writer Michael E Buerger making reference to "The Cuckoo's Nest", a strategy involving "the use of an abandoned, temporarily vacant, or on-loan premises for retail drug sales", in his article 'Defensive Strategies of the Street-Level Drug Trade' (published in a 1992 issue of the Journal of Crime and Justice).
The term then came into more widespread use more recently – in 2010, the Guardian published an article with the headline "Vulnerable tenants targeted by drug gang 'cuckoos'" which detailed how gangs were "befriending vulnerable people before taking over their homes for crack dealing".
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Independent UK charity Crimestoppers suggests that victims of "cuckooing" are "often drug users but can include older people, those suffering from mental or physical health problems, female sex workers, single mums and those living in poverty."