The process of putting a collar on something, or in informal speech, the act of catching or arresting someone.
From the verb 'collar' (to seize by the collar, from 'collar' the noun) + '-ing' gerund suffix. The figurative meaning (capturing someone) emerged in 17th-century slang because grabbing someone by the collar was an arrest tactic.
The slang meaning 'to catch or arrest' comes from literally grabbing someone by their clothing collar—this is why 'collar' became code for law enforcement, giving us 'blue collar' (working class workers who wore sturdy collars) versus 'white collar' (office workers in dress shirts).
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