Thais are given a nickname by their family at a very young age, and this is what family, friends, and familiar associates call each other. This name is NOT a legal name, and does not exist anywhere in legal paperwork.
Thais are given a legal first name and surname at birth, but no middle name. When you first know a Thai person in an office or other fairly formal setting, then you address them as Khun [Firstname]. Later, this may switch to their nickname.
However, in very informal settings, Thai people will introduce themselves by their nickname only, and you may never know their real name unless you ask.
The nicknames are usually one syllable words though some are two syllable and sometimes three. Their translations mean silly things like “shrimp”, “baby chicken”, “eyes”, “gift”, and lots of other things.