The word "careful" means being marked by prudence or concern, and more fully, showing close attentiveness to avoiding danger and trouble; an antonym of it would therefore be a word such as "careless," according to the Merriam-Webster online dictionary. An example of "careful" used in a sentence is "The police were careful not to taint the investigation scene." An antonym is a word that is the opposite in meaning of another word, so it makes sense that the opposite of careful, or being full of care, is a word that means not paying close attention, or expressing little to no care.
Here are some other words that are antonyms of "careful," according to the Merriam-Webster online dictionary:
Here are a few words the same dictionary identifies as near antonyms of "careful":
Thesaurus.com, which is based on the 2013 "Roget's 21st Century Thesaurus, Third Edition," includes these additional antonyms for "careful":
The online Macmillan Dictionary expounds a bit on the antonym "careless," stating that it is when a person is not thinking or worrying enough about something, such as an administration that may be careless about the freedom of the press. At the same time, another definition could be applied, as in "a careless laugh," which invokes the sense of a natural or relaxing laugh.