What term describes a carefree and nonchalant attitude, often dismissive in an offhand manner?

The term that best describes a carefree and nonchalant attitude, often dismissive in an offhand manner, is "cavalier." This word is used to characterize someone who shows a lack of concern or regard for important matters, often acting in a dismissive or unconcerned way. When someone is described as having a cavalier attitude, it implies that they approach situations with a sense of carelessness or disregard for potential consequences, reflecting a casual or flippant stance.

In contrast, the other terms do not capture this particular attitude. "Complacent" refers to a sense of self-satisfaction, often without awareness of potential dangers or deficiencies, which doesn’t necessarily express a carefree or dismissive attitude. "Clandestine" relates to actions that are secretive or concealed, particularly in a deceptive manner, and does not convey a sense of nonchalance. "Cogitate" is about thinking deeply or pondering over a subject, which is opposite to the idea of being carefree and dismissive. Thus, "cavalier" is the term that accurately embodies the described attitude.