na-adjective, noun
negligent; careless; halfhearted; irresponsible
Describes an attitude or action that is sloppy, lacking proper care, or done without commitment. Often used for work, attitude, or responses that are offhand or reckless.
彼の投げやりな仕事ぶりにはがっかりした。
I was disappointed by his negligent way of working.
そんな投げやりな態度では成功
You can't succeed with such a halfhearted attitude.
Irregular kanji usage; 槍 is a phonetic substitute, not semantically related.
From 投げ (throwing) + やり (doing), literally 'throw-doing', implying doing something in a careless, throwaway manner. The exact historical derivation is uncertain, but the compound suggests an attitude of tossing something aside without proper care.