Definition
To be defensive or hypervigilant; to adopt a defensive posture; to avoid conflict
How People Actually Use It
To go on the defensive. To make oneself incapable of harm or damage. Can be used in a literal or figurative sense.
"1. When losing the game, he chose to turtle up, giving himself an opportunity to comeback. 2. The opposing boxer decided to turtle up and wait for the striker to tire himself out before striking back."
Source: Urban Dictionary