idleness; lack of diligence; lazy study habits
Refers to a lack of effort in studying or working, often used self-critically or to describe someone's attitude.
I failed the exam because of my own lack of study.
He is lazy about studying and always forgets his homework.
勉強不足 is a more common and straightforward way to say 'insufficient study', while 不勉強 sounds slightly more formal and self-critical.
Compound of 不 (fu, 'not') and 勉強 (benkyō, 'study'). The exact historical derivation is uncertain, but the word follows a common pattern of negating a noun with 不.