spy on
睪 centers on covert observation: spying or watching secretly. The given meaning is narrow, so treat it as a specific act of surveillance.
The historical origin of 睪 is uncertain. The modern form visually combines ⺫ (a net-like component) with 幸, but this does not clearly explain the meaning 'spy on'.
The top ⺫ looks like a net or eye-like grid, and the bottom 幸 suggests a person. Picture a hidden observer using a net-like screen to spy on someone without being seen.
For エキ, imagine a spy peering through a keyhole and exclaiming 'Eek!' when almost caught: eek -> エキ, tied to the act of spying.