It is an acronym for "Anti Nicotine and Tobacco Zealot"
These are the people who hate nicotine and tobacco so much that they refuse to acknowledge the benefits of tobacco harm reduction. They are complete tobacco and nicotine prohibitionists, not just anti-smoking. They deny and lie that low-risk forms of smokeless tobacco, such as snus, and e-cigarettes aren't up to 99% less hazardous than smoking. They insist that you haven't really quit smoking unless you have quit all forms of nicotine. They'd rather see smokers keep smoking (quit-relapse-quit-relapse) until they get brow-beaten into (hopefully) quitting rather than see them continue to use a low-risk source of nicotine. They'd ban e-cigarettes just because they LOOK like smoking. They follow the "quit or die" mantra.
Essentially, they are so zealous in their hatred of tobacco and all forms of recreational nicotine that they actually put smokers at greater health risks by denying that there are safer forms of tobacco and nicotine.
The term is also used, in general, to mean any person or group who irrationally wants to ban or dissuade smokers from switching to reduced harm alternatives such as snus and e-cigarettes.
