If "purpose" refers to a reason or goal:
* Meaninglessness: This emphasizes the lack of any defined goal or direction.
* Pointlessness: This suggests that there's no value or significance in what is being done.
* Futility: This implies that the action is ultimately unsuccessful or ineffective.
* Randomness: This indicates that there's no underlying plan or intention behind the action.
If "purpose" refers to a sense of direction or meaning in life:
* Aimlessness: This describes a lack of goals or aspirations.
* Drifting: This suggests a lack of direction or purpose in life.
* Nihilism: This is a philosophical view that denies the existence of inherent meaning or value.
Other possibilities:
* Accident: This refers to something happening without intention or planning.
* Chance: This implies that something happens without a predetermined purpose.
The best antonym will depend on the specific context in which "purpose" is used.