Webby does a "good job" of starting off this node about urine, but sig is more than just your run-of-the-mill pee.
"Sig" is the old name for old urine, or "aged" urine used in the manufacture of woolen articles of clothing. When wool was plucked from the sheep and spun into yarn it was discovered that the grease interfered with weaving. The process of de-greasing and softening wool is called fulling, and fulling requires that the wool be fully scoured. Wool can be scoured with soft water, naturally occurring alkalis washed from the ashes of plants, or stale urine. The alkali of the urine (ammonia, ammonium hydroxide, and ammonium carbonate) is the least damaging of all alkaline substances to wool, and it was usually the easiest and cheapest of the choices to procure centuries ago.
Using urine in the fulling of wool was the most popular method, and therefore the urine was carefully preserved by every means that could be adopted. The factories used to supply to any household who would receive it and undertake the task a tub or vat for the purpose of storing their liquid waste; moreover, these households were paid an annual sum for doing so. Each textile establishment then kept a large barrel on wheels, drawn by a horse, which used to make regular rounds to collect the contents from its clients. The payment for producing and keeping one's own urine was small, but in light of the alternative choice of what to do with it, many households in the vincinity of a textile factory were more than glad to part with their urine for what was a modest fee.
Today, purpose-made chemical detergents do the job slightly better but, until quite recently in the 19th century, sig was the agent of choice for preparing wool.