Which word refers to something flourishing or spreading unchecked?

The term that refers to something flourishing or spreading unchecked is "rampant." This word often describes a negative situation where growth or expansion occurs without control, frequently associated with weeds overgrowing or problems becoming widespread. It conveys the idea of something being unrestrained and occurring in a vigorous or unmanageable manner.

In contrast, "robust" typically means strong and healthy, particularly in the context of physical health or systems. "Feckless" characterizes someone lacking in initiative or effectiveness, often used to describe incompetence or irresponsibility. "Scathing" refers to a harsh or severe criticism, usually directed at someone's actions or character, rather than a notion of growth or spread. Thus, "rampant" is the most fitting choice for the concept of flourishing or spreading unchecked.