The idiom "不知所措" is a classical four-character compound, or chengyu, formed through a grammatical structure where "不" (bù, meaning "not") negates the following verb phrase "知所措" (zhī suǒ cuò). This verb phrase itself is a fixed construction from Classical Chinese: "知" means "to know", "所" is a particle that turns the following verb into a nominalizer (like "that which"), and "措" means "to arrange", "to handle", or "to deal with". Thus, the literal meaning is "to not know that which to do" or "to be at a loss as to how to handle the situation". It figuratively expresses a state of being utterly bewildered, panicked, and unable to react appropriately because one is overwhelmed and does not know what course of action to take.