Refers to acting without careful thought or consideration, often leading to careless mistakes or inappropriate remarks.
His thoughtless remark angered everyone.
Thoughtless actions lead to regret.
不注意 means carelessness or inattention, often about physical mistakes; 無思慮 is more about mental thoughtlessness or indiscretion.
Compound of 無 (mu, 'without') and 思慮 (shiryo, 'thought, consideration'). The exact historical derivation is uncertain, but the word follows a common Sino-Japanese pattern for abstract nouns.