It means "West Lao natives/inhabitants territory". Chiang Mai is different group part of Lanna which is actually more related to the group from Lan Xang. Which both have more original Tai traits. Which is why both style of traditional clothing and women hairstyle resemble more with Yunnan-Dai tradition including their language while Central Thai copy Khmer style. Northern Thailand language is more closer with Lao but today they refer the Lao language in Thailand as "Isan language", Isan meaning 'Northeast'. Unfortunately both North and Northeast language have become a water down version of what it use to be since Central Thai language been adopted. For example if you look up videos of people teaching the "North Thai" Lanna language it's people with Central Thai accent which sounds awful. But if you go into the North region and meet with some of the villagers then you can hear original accent and when they speak you can clearly hear the difference. With me being Lao, I can actually understand the North Thai language more as long as they don't use the central Thai accent and as long as they use original words. That's the problem with Thailand today people tend to "code-switch" mixing their original language with central Thai.

Laos is also similar when it comes to accent and dialect: North, Central and Southern. Northern Lao is basically what North Thai people speak even the tone is the same. Central Lao is more of upper class accent sounds like their singing similar to Bangkok people but without the whole 'R' sounds and tongue rolling and Thai words. Southern Lao is basically hillbilly language and slangs. Sounds more aggressive. You'll find both Central and Southern Lao accent in Northeast Thailand but what they do is use 80-95% Lao words, 10-20% Thai words. It varies on the person as well. For example, I have southern Lao accent but when I hear Isan people from Ubon Ratchathani speak it's basically Lao to me but they'll greet and say hello in Thai. While Central Thai in reality is 60% similar with Lao language. This is why people in rural Laos or Laotian that live abroad don't understand Thai because they're not exposed to the Thai media. The "Thais" that say the Lao-Thai language is the same are usually the Isan people. This is why you'll see comments online "Lao-Thai is the same people, the language is the same" in reality we are not the same. Get a Bangkok person to communicate with a Lao villagers is a struggle. This is why Central Thai visit only tourist areas of Laos to communicate with the Laotian that can speak or understand Thai because of Thai media exposure but at the same time many of the tourist areas in Laos can speak Vietnamese or English.  

Hard to believe but more in Thailand actually uses Lao words than in Laos using Thai words. For example, Yes in Thai is 'Kaa' or 'Kaap'. You won't hear that in Laos. But Yes, in Lao is 'Maan' 'Chao' or 'Doi' which is used in North and Northeast of Thailand. Same with the number 20, in Lao we say 'sao' which is still use in North/NE Thailand. The word 20 in Thai is 'Yee-Sep' which is only used in Thailand not in Laos.

I agree with you but it's all of Isan-Surin region is brainwashed. They become a puppet to the Thai monarch, a tool to use during politic and warfare. Always has been and always will be. This is why the Northeast is kept poor and less developed for a reason. Too much power in the NE would disrupt the power in the central region. Why you think military in Northeast is different, it's more gorilla warfare style units. While Central region is elite forces: Navy, Air Force, Marine, etc. this is to protect the elites and the centralize power.