Wiktionary
tailor's ham
n. (cx sewing English) A tightly stuffed pillow used as a curved mould when pressing curved areas of clothing.
Wikipedia
Tailor's ham
A tailor's ham or dressmaker's ham is a tightly stuffed pillow used as a curved mold when pressing curved areas of clothing, such as darts, sleeves, cuffs, collars, or waistlines. Pressing on a curved form allows a garment better to fit body contours. To accommodate tapering or garments of different sizes, it has roughly the shape of a ham.
Usage examples of "tailor's ham".
Stitch something perfectly smooth, then pull it too hard over a tailor's ham, and it bulges and puckers.