Verifiable evidence has been provided from 1986.
In the United Kingdom, after a bar or pub’s doors are closed at the official or legal closing time, customers already inside are sometimes allowed to stay and continue drinking for a period of time; this is called a lock-in. The noun lock-in is attested from the late 19th century in other senses, but the earliest evidence found for this particular use is from 1991:
Here is a studio where the phrase ‘lock-out’ is heard less often than the phrase ‘lock-in’, referring to the after-hours drinking policies of the local pubs.
1991 Independent 5 Dec., p. 20/3
The elder members of the OED’s staff know from personal experience that this practice existed before 1991, but we have been unable to find earlier verifiable evidence of this term for it. Can you help us find earlier evidence of lock-in referring to a period after closing time in a bar or pub when customers already inside are allowed to continue drinking?
Posted by OED_Editor on 17 September 2015 19.58