What is the meaning of Microchip's acronym TRIS for data direction registers?

TRIS stands for TRIState. It means the port is waiting for an input rather than output a high or low signal.

It's named as such because a port can have 3 states:

• Output High
• Output Low
• Input (High Impedance)

The term "tri-state" is often used informally to refer to the act of floating a pin; I know that National Semiconductor used to have a trademark on the term, though its widespread usage as a common term may have rendered it unenforceable.

Incidentally, in case anyone's curious why the OPTION instruction is so called, and the settings aren't controlled via a register, on the original General Instruments PIC devices, there were no TRIS or OPTION instructions; instead, the selection of which pins should be floating, as well as the settings for the timer/watchdog prescalar and modes, were part of the factory specification (the devices used mask ROM, so the whole program would be a factory spec).