Noun: Short form of 'off licence'.
A British pub has an 'on'
licence (licenced to sell intoxicating liquor for consumption ON the
premises). They used to also have a little hatch where you could buy drink to take away and this had an 'off' licence. All such purchases had to be consumed off the premises.
Nowadays Britons buy their
take-out booze in a shop which is unconnected to any pub. These shops have to apply for the same 'off licences' from local magistrates and have retained the name.