The misnamed Thai whisky (lao) refers to a number of liquors. The best known are the infamous Mae Khong (แม่โขง "Mekong") brand and its competitor, the sweeter Saeng Som ("Sangsom"), which are both brewed primarily from sugarcane and thus it is actually rum. Indeed, the only resemblances to whisky are the brown color and high alcohol content, and indeed many people liken the smell to nail polish remover, but the taste is not quite as bad, especially when diluted with cola or tonic water. This is also by far the cheapest way to get blotto, as a pocket flask of the stuff (available in any convenience store or supermarket) costs only around 50 baht.
The "real" Thai whisky is lao khao (เหล้าขาว "white liquor"), which is distilled from rice. While commercial versions are available, it's mostly distilled at home as moonshine, in which case it also goes by the name lao theuan ("jungle liquor"). White liquor with herbs added for flavor and medical effect is called ya dong (ยาดอง). Strictly speaking, both are illegal, but nobody seems to mind very much, especially when hill-tribe-trekking in the North you're likely to be invited to sample some, and it's polite to at least take a sip.