WooHooium is a type of metal that was added in The Sims 3: Ambitions expansion pack. It can be obtained by mining, sculpting, digging through trash, getting pet to dig in dirt, and exploring catacomb.
Although little is known about it at the moment, it is said that taking a WooHooium ingot and putting it on either side of a double bed means that no WooHoo will ever be rejected. Little hearts appear from the ingots when the WooHoo is in progress. This metal looks different from other metals; WooHooium has a heart on it in the space (on the ingot itself) where most metals in The Sims 3 have a PlumbBob.
A fairly easy way of obtaining WooHooium is by failing to sculpt a stone sculpture. However, it is not a reliable way, as the breaking of the stone sculpture itself is random, and Woohooium is just one of the metal/gems that can be acquired through this method.
It is possible for a dog with a high hunting skill to use "sniff out collectible" and thereby obtain Woohooium randomly. While this method is also not perfect, all the gems/metals/rocks received in the interim make this method, if not always successful, at least profitable.