“Not my circus, not my monkeys” is a phrase people often use when they emotionally step out of chaos, dysfunction, or other people’s drama. Psychologically, it reflects:

• boundary setting,
• emotional detachment,
• nervous system self-protection,
• and refusing responsibility for problems created by others.

After long periods of conflict or emotional overload, the brain often shifts from:

• hypervigilance and trying to fix everything,
  to:
• conservation of energy and psychological distance.

In neuroscience terms, chronic stress can keep the threat system activated for months or years. Eventually many people reach a point where the nervous system simply says:

“I cannot carry this anymore.”

The phrase is not necessarily about not caring. Often it means:

• “I recognise the dysfunction,”
• “I see the patterns,”
• but “I no longer want to participate in managing them.”

That can actually be a sign of recovery from enmeshment, trauma bonding, or chronic emotional responsibility for other people’s behaviour.

There is also an important difference between:

• setting boundaries,
  and
• becoming emotionally numb.

Healthy detachment says:

“I will protect my peace and let professionals or responsible adults handle what is theirs.”

That is very different from denial or indifference.