What is “metaphysico-theologo-cosmolo-boobology” ?

Candide is a satire, and the term "metaphysico-theologo-cosmolo-boobology" is Voltaire's way of poking fun of and mocking abstract philosophy. This specific wording is invoked in reference to Voltaire's introduction of Pangloss, who is himself a mocking caricature of Leibniz. This should give you some idea as to how Voltaire, at least within the context of Candide, views abstract philosophical inquiry.
Each of these terms (save for the last) refers to specific topics of abstract inquiry. Metaphysics is one of the cornerstones of philosophy, and it tends to point toward abstract questions as to the fundamental nature of reality and existence. Theology involves questions as to the nature of God. Cosmology refers to the study of the universe. Candide throws each of these three terms together into a caricature of philosophical inquiry, which is presented as ungrounded in reality and, speaking in practical terms, ultimately worthless.
This is reflected in Pangloss's teachings. Pangloss is presented as the finest philosopher of his age, with a reputation for great wisdom, and yet his philosophy is all nonsense. Furthermore, as if the emptiness of his reasoning is not sufficient in and of itself, his vision of reality is actively at odds with the reality which Voltaire's Candide represents. Pangloss's vision of the universe defines it as "the best of all possible worlds" (in this particular phrasing, Voltaire is referencing Leibniz), but of course, the world which Voltaire presents is rife with injustice and cruelty. Thus, when viewed within the context of the world Voltaire presents, the very idea that this could be "the best of all possible worlds" isn't just vapid abstract reason—it is a deception. That the world can be better than it currently is should be practically self-apparent given how dysfunctional everything is.
The term "metaphysico-theologo-cosmolo-boobology" must be understood as a term of mockery applied to abstract philosophy and particularly to Pangloss's ludicrous and ultimately irresponsible brand of it. Voltaire views this entire reasoning as nonsensical, and this particular phrasing is a reflection of that larger attitude.