Careless means not paying enough attention, so things get broken and mistakes get made. A careless spelling error in the first line of a letter, a careless driver who checks the mirror after changing lane.
It sticks to the person and to the thing they did: a careless student, a careless mistake, careless driving, which is a real charge in British law. With OF or WITH it means not caring at all — careless of the danger, careless with money. Do not read it as the opposite of caring: someone careless is sloppy, someone uncaring is cold, and those are different failings. Poems and older novels use careless for a happier thing, free of worry, and you can still hear it in careless laughter. The noun is carelessness and the adverb is carelessly.
1adjectivecarelessNot giving enough attention to what you are doing, so mistakes and accidents follow.
“One careless moment with the knife and the evening ended in hospital.”
2adjectivecaringNot caring about something — used with of or with.
“He was careless of his own safety and everyone else's.”
3adjectivefreeFree of worry; light and unbothered.
“There was a careless ease about the way she played.”
Careless is care plus less, and it is old: Old English carleas. But it meant free from care — untroubled, without a worry — back when care itself meant sorrow. The modern sense is the same shape read the other way round: not without worry, but without taking care.
Old English
carlēas
Middle English
careles
English, today
careless
From Middle English careles, from Old English carlēas. From Middle English careles, from Old English carlēas (“careless, reckless, void of care, free from care, free”), equivalent to care + less. Cognate with Icelandic kærulaus (“careless, negligent”).
Etymology from Wiktionary, CC BY-SA