Siamese-a native or inhabitant of Thailand. Tai, Thai. Kingdom of Thailand, Siam, Thailand-a country of southeastern Asia that extends southward along the Isthmus of Kra to the Malay Peninsula; Thailand is the official name of the former Siam.i•a•mese. 1. of or pertaining to Siam or its inhabitants. 2. twin; closely connected; similar. 3. twofold or two-way: a Siamese sprinkler. 4. a native or inhabitant of Siam. 5.