Not being used appropriately; not occupied; (of time) with no, no important, or not much activity.
My computer hibernates after it has been idle for 30 minutes.
2009, Jane Bryant Quinn, Making the Most of Your Money Now:
The majority of accounts require no minimum balance and charge no monthly service fee. Where monthly fees and balance requirements exist, they're low. You earn no interest on the idle money in the account.
Molly the dairymaid came a little way from the rickyard, and said she would pluck the pigeon that very night after work. She was always ready to do anything for us boys; and we could never quite make out why they scolded her so for an idle hussy indoors. It seemed so unjust. Looking back, I recollect she had very beautiful brown eyes.
“I don't mean all of your friends—only a small proportion—which, however, connects your circle with that deadly, idle, brainless bunch—the insolent chatterers at the opera, the gorged dowagers, the worn-out, passionless men, the enervated matrons of the summer capital,[…]!”