The word "牢笼" is formed by combining the characters "牢" and "笼". "牢" originally means an enclosure or pen, such as for livestock, and extends to denote a prison or something firm. "笼" refers to a cage or basket used for confining objects, like birds. Together, they create a compound word where both characters contribute semantically related ideas of confinement, resulting in a term that literally means a cage or enclosure and metaphorically signifies imprisonment or entrapment.