Sizing
Coating the warp threads with starch paste so they are strong and smooth enough to survive the rubbing of weaving.
Also: starching the warp, kanji, warp sizing
Warp threads are stretched tight and are rubbed again and again by the reed and the heddles, so fine and loosely twisted yarns can fray or snap. Before weaving, the warp is sized: it is brushed with a thin paste, usually of starch such as rice or a similar flour, and dried. The coating glues down the loose fibre ends and makes the thread smoother and stronger. After weaving, the cloth is washed and the starch is rinsed out. This is why a new cotton saree can feel stiff and soften only after the first wash.