The well is the shelf under the bar where they keep liquor at hand for mixing drinks. So, a well brand or house brand is the brand of liquor served when a customer requests a drink by its generic name (e.g. gin and tonic, scotch and soda). It's usually the least expensive brand served, as opposed to more expensive call brands.
Order me a Jack and Coke. Their well brand tastes like bathtub gin.
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