The term probably has US origins, and is used in connection with prostitution, expecially in South East Asia. It covers all and any sexual services in return for money. This can be massages, escorts, soapies, the asia bar scene or GFE.
A term frequently used in the South East Asia P4P scene to mean Tour Guide or Thai Girl.
1: Tour Guide, a term originated in the Philipinnes for a girl who accompanied a guy for the duration of his holiday/R&R. Probably an US servicemen originated term to differentiate from short-term or one-nighters. The young lady usually exchanges for her time for for money. A TG is usually thought of as a rentable girl-friend rether than prostitute.
2: Thai Girl. In Thailand, though the meaning can be "tour guide", it is more often used as Thai Girl - she may or may not be part of the P4P scene ... it can refer to girlfriend or wife.