Here are some possibilities:
* Timbuktu guitar: This could be a specific brand or model of guitar named after the city in Mali.
* Timbre guitar: Timbre refers to the tone color or quality of a sound. Perhaps it's a guitar designed for a specific timbre.
* "Tim" as a nickname: Someone might refer to their guitar as "Tim" as a nickname.
* A typo: It could be a typo of a different guitar term like "tin guitar" or "trim guitar."
Without more context, it's difficult to say for sure what "Tim guitar" means. If you have more information, please provide it so I can help you understand.