A 1UP user passed along the suggestion that we check the patent records for the name 'Sixaxis' and see what turned up. Scanning the trademark's text it's clear that this could be any number of things (see image above).
However, the term "six axis" has appeared frequently in Sony PR, including multiple times in the May 8, 2006 announcement of the controller's motion sensing functionality:
The controller for PS3 employs breakthrough technology of high-precision, highly sensitive six-axis sensing system that does not require any devices other than the controller itself for seamless interactive operation, thus eliminating additional settings to TVs. With this technology, ways to enjoy PS3 will be further enhanced by accessing PS3 through the network, while retaining the six-axis sensing capability.