taru-adjective, adverb taking the 'to' particle
aimless; rambling; desultory; casual
Describes doing something without a clear purpose, plan, or focus, often with a sense of drifting or being haphazard. Used as a たる adjective or adverbially with と.
He was wandering aimlessly through the streets.
It's a waste to spend your days in a desultory manner.
If you just listen to the class aimlessly, you won't really absorb the material.
His superior finally spoke up about his listless, unfocused attitude.
Without any goal, I was just drifting through my days aimlessly.
漫然 describes doing something aimlessly with no clear plan, whereas 計画的 describes doing something in a deliberate, planned way.
漫然 describes acting without a clear purpose or direction (aimlessness), while 漠然 describes a lack of clarity in thought or expression (vagueness), and the two 然-words are easily mixed up.
From 漫 (man, 'rambling, aimless') + 然 (zen, a suffix forming adjectives/adverbs indicating a state). The exact historical derivation is uncertain, but the word has been used in literary Japanese since at least the Meiji period.