The phrase "bulging eyes" refers to a physical appearance where one's eyes appear protruding or popping out due to some underlying condition. It could be caused by certain medical conditions such as Graves disease, which is an autoimmune disorder that affects the thyroid gland and can cause bulging eyes along with other symptoms like weight loss, anxiety, irritability, tremors, heat intolerance among others. In some cases, it could also be caused by certain medications or drugs, eye infections or injuries, or even a reaction to stress or fatigue. The phrase can also be used figuratively to describe someone who is very surprised or shocked about something and their eyes appear wide with surprise.