Too hard to control, handle, or keep in order. You use it when something has grown past the point where a normal amount of effort is enough — a class of forty children when you are one teacher, hair that will not stay flat, a pile of work that is now so big you cannot even start it, debt that has passed the point where paying a little each month helps. The feeling inside the word is not 'bad' but 'past my control': the thing has its own momentum now. It is used of people too — an unmanageable child, an unmanageable crowd — and there it is a little bit gentler than 'badly behaved', because it says nothing about whether the person is at fault, only that whoever is in charge cannot cope. Note that it points at the controller as much as at the thing controlled: an unmanageable workload is unmanageable for someone. That is why people often soften it into a complaint about a situation rather than a person — 'the schedule has become unmanageable' is a way of saying 'I cannot do this' without saying I. It is a fairly plain, everyday word, common in work and news writing, and slightly formal in speech; in casual conversation people are more likely to say 'out of control' or 'too much'.