Reply #66: One definition (from SkepticWiki)

[BiggJawn]

Woo-woo (or sometimes, simply woo) is used within parts of the skeptical community in referring to extraordinary beliefs for which it is felt there is insufficient extraordinary evidence, and
people who hold those beliefs.

Sometimes used as an adjective ("My brother has a lot of woo-woo beliefs"), other times as a noun ("That message board is full of woo-woos"), it is almost always used as a term of derision.

Origin

The origin of the term is unclear.

One theory is that it comes from the "woo-ooo" sound made by a Theramin, the electronic instrument often used in old horror films to emphasize that something strange or mysterious was happening (such as the appearance of a ghost or alien). Another theory is that the term woo-woo comes from the theme song of Rod Serlings's The Twilight Zone. Yet another theory is that "woo woo" was early 20th century slang for insanity.