to buy a large amount of something so that you will have enough for future use.
When there's a storm coming we always stock up on food and candles.
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stock up on (something)
To gather, build up, or hoard a supply of something. We stocked up on bottled water and canned goods, so we should be fine during the storm.Don't worry, I've been stocking up on fuel for the past three years for just such an occasion.