The word "大大咧咧" is formed by reduplicating the character "大" (meaning big or, in this context, careless) to create "大大", and then adding the colloquial suffix "咧咧", which itself is a reduplication of "咧". This combination results in an adjective that describes a person's demeanor as straightforward, unconcerned, or negligently casual, with the repeated elements emphasizing the continuous or heightened state of this trait.