Maybe from UK ‘off-license’?
Re:A Typology of Canadianisms
by goodcanadian on Thursday July 10, 2025 @01:57PM
(#44523691)
Similar concept, I suppose. When I was young, alcohol for consumption at home was generally only available for purchase in government run liquor stores. (This is still the case in some provinces, but no longer true in Alberta.) However, a few licensed premises (bars and restaurants) were permitted to do off-sales and sell alcohol that you were allowed to take with you off the premises.
Off-sale has long been used in Alberta. I have a memory of asking my parents what it meant when I was a kid (and I am in my 40s, now).