As best I can tell, Tim Negris is the man who coined the term Thin Client when he was with Oracle. It originally described scaled down applications that ran in conjunction with a full version of an application that was loaded on a server.
When Windows OS versions that allowed multiple users came on the scene, one of the first hardware companies to start selling client hardware was Wyse with the introduction of their WinTerm in 1995. This made sense – Wyse was already a huge seller of “dumb” terminals for mainframes and providing a Windows Terminal was logical.