Obstreperous

obstreperous [ŏb-STREP-ər-əs]
adj. 1. resisting control or restraint in a difficult manner; unruly. 2. noisy, clamorous, or boisterous

[From Latin obstreperus ("clamorous") from ob- ("over") + strepere ("to rattle)]

Perhaps the biggest problem with obstreperous is it can mean different things, but it's not always clear which one is intended. The definitions tend to bleed into each other. Although its etymological origins are not in an unruly mob, that is where you should place this word. An unruly mob resists control. It's noisy and clamorous and boisterous. My recommendation is to restrict your usage of obstreperous to mean "unruly, verging on out of control." If a group of people is having a good time, but is noisy and enthusiastic, I wouldn't classify them as obstreperous, though perhaps they could be called boisterous. When a wild child is throwing a fit, crying, tossing his toys about, unable to control his emotions, that's obstreperous.

Here's an example from The Times of London where it's not clear how obstreperous is being used. Does the writer mean that taxi drivers are unruly? or merely noisy?

After trying to hide holes in their socks while shuffling through airport security, hunting a seat in the departure lounge, tipping red wine on to documents on a crowded flight and dealing with obstreperous taxi drivers on arrival, the experienced traveller will find it hard to empathise with the beauty queen’s innocent enthusiasm (for travel).

In this instance, George Will uses obstreperous as a synonym for clamorous ("expressing vehement dissatisfaction"). That's technically acceptable. However, clamorous would have been better, because I doubt the people he describes were really unruly, verging on being out of control:

Storing nuclear waste, which decays very slowly and emits great heat while doing so, has been studied since 1955, when nuclear submarine propulsion technology was adapted to generate electricity. After considering storage on the seabed or a remote island or in the polar ice sheets, or rocketing the waste into orbit around the sun, the government settled on deep geologic storage as the preferred solution. Some Kansas salt mines were considered, but the mines were too difficult to seal and, besides, Kansas became, as Nevada is now, obstreperous.

When obstreperous is used as a synonym for unruly, there's no confusion. That's how it is used most often, as well. Here is an example of that from a court case called, CITY OF VIRGINIA BEACH v. MILES STEPHEN SMITH:

Mr. Smith is charged under Virginia Beach’s disturbing the peace ordinance, Code Sec. 23-10. which forbids “any person to disturb the peace of others by violent, tumultuous, offensive or obstreperous conduct or by threatening, challenging to fight, assaulting, fighting or striking another.”