—used to express joy, approval, or excited enthusiasm
NASA's social networking presence took off with the Mars Phoenix Lander, which tweeted, "Are you ready to celebrate? Well, get ready: We have ICE!!!!! Yes, ICE, WATER ICE on Mars! w00t!!! Best day ever!!" on June 19, 2008. — Amanda Stratford, Florida Today (Brevard County, Florida), 16 Oct. 2008
—often repeated
I know of some organizations whose events exceed their budgeted goal (woot woot!) and others who hope to break even and must decide if it's worth repeating. Alyce Lee Stansbury, Tallahassee Democrat, 6 Apr. 2014
Word History
Etymology
perhaps extension of whoo entry 1, with t representing glottal closure