Real Dictionary

idle
If machines or factories are idle, they are not working or being used.
If people who were working are idle, they have no jobs or work.
adjective: If you say that someone is idle, you disapprove of them because they are not doing anything and you think they should be.

idle in American English
having no value, use, or significance; worthless; useless
adjective: not working or active; unemployed; doing nothing
intransitive verb: to pass time doing nothing
noun: the state or quality of being idle
transitive verb: to pass (time) doing nothing

idle in British English
adjective: unemployed or unoccupied; inactive
verb: to waste or pass (time) fruitlessly or inactively

idle in Mechanical Engineering
verb: If an engine or shaft idles, it turns without doing anything useful, for example moving a vehicle forward or making another part move.