The word "hobby" refers to an activity or interest that one does for pleasure in one's spare time.
As there isn't a traditional exact antonym for "hobby", we can consider words that are somewhat in opposition in terms of concept. For example, "chore" or "duty".
A chore is a task that one has to do, often out of necessity rather than for pleasure. For instance, doing the laundry or washing the dishes is a chore. Unlike a hobby, which is chosen voluntarily for enjoyment, chores are often not looked forward to and can be a source of stress rather than relaxation.
A duty is an obligation or responsibility. For example, paying taxes or fulfilling work responsibilities is a duty. While hobbies are done for personal satisfaction, duties are often imposed by external factors and must be fulfilled regardless of one's personal preference.
In conclusion, although there is no perfect antonym for "hobby", words like "chore" and "duty" represent activities that are the opposite of hobbies in terms of motivation and enjoyment.