Which word describes a lack of wisdom?

The word that describes a lack of wisdom is "foolish." This term directly connotes a lack of good sense, judgment, or prudence, embodying the essence of wisdom's absence. When someone acts in a foolish manner, they often make decisions that are not only unwise but can also be potentially harmful or nonsensical.

While "impolitic," "unwise," and "injurious" can imply a lack of wisdom in certain contexts, they do so in more nuanced or specific ways. "Impolitic" refers to actions that are unwise in a political or social sense, often lacking tact or discretion. "Unwise" is a more general term indicating that a decision or action is not wise; however, it may not carry the same connotation of negative judgment that "foolish" does. "Injudicious" similarly means lacking judgement or wisdom, but it does not have the same playful or pejorative nuance as "foolish" does. Therefore, "foolish" is the most direct and comprehensive word to describe a lack of wisdom.