The wai means respect. It's not a general friendly greeting.
Much like a military salute, the wai is more than a greeting, it is a gesture of respect offered by the lesser to his elder or superior. Every Thai seems to automatically sense and known his place in society versus everyone else around him. Exactly who pro-offers the wai and who answers it is extremely important. This is an aspect of Thailand that few foreigners can ever fully understand. It is generally best for foreigners to refrain from wai-ing at all. However, a pro-offered wai should always be acknowledged. It is perfectly acceptable to answer a wai with a verbal greeting (sawadee, kap), a bob of the head or a gentle wave of the hand. The hand wave is particularly effective in that it says don’t wai me. I’m just an ordinary guy.
Generally speaking, there are very few instances where a foreigner would pro-offer a wai. Inexperienced foreigners tend to wai everyone. This generally brings on more ridicule than respect. For example, an adult never wais a child, even in reply. Nor should a foreigner reply with a wai to service personnel like a hotel doorman, maid, waiter or other workmen. At the same time, all such wais should be acknowledged in some way. A simple smile or nod of acknowledgment is often sufficient.
At the same time, every foreigner should be aware of those special situations where he should answer a wai with a wai of his own. For example, the Thais generally show extreme respect to the elderly. Thai society is replete with exchanges that acknowledge the older/younger, greater/lesser, pee/nong status of the meeting pair.
One situation where a foreigner should answer a wai with a wai is when it is pro-offered by someone clearly older. The older Thai is acknowledging the foreigner’s exalted status.
(The photo is from the book titled
The Katoeys of Nana Plaza.)