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means A solid fat (like lard, butter, or vegetable fat) worked into flour to coat the gluten strands, keeping baked goods crumbly and tender rather than tough.

from Straightforward kitchen English: it 'shortens' the gluten strands in dough. In baking, gluten strands that stretch long and elastic make bread chewy; fat coats and 'shortens' those strands, physically interrupting them so pastry breaks into crumbly, tender bits instead. The word 'shortbread' preserves this exact logic — it's simply bread made short by fat. The product name 'Crisco' (short for 'crystallized cottonseed oil') later became a 20th-century byword for shortening itself, after companies began selling purified vegetable fat as a cheaper substitute for lard.