What is "to be in mischief"

Mischief \Mis"chief\ (m[i^]s"ch[i^]f), n. [OE. meschef bad result, OF. meschief; pref. mes- (L. minus less) + chief end, head, F. chef chief. See Minus, and Chief.]

1. Harm; damage; esp., disarrangement of order; trouble or vexation caused by human agency or by some living being, intentionally or not; often, calamity, mishap; trivial evil caused by thoughtlessness, or in sport.
   --Chaucer.

   Thy tongue deviseth mischiefs.
   --Ps. lii.

2. The practice whereof shall, I hope, secure me from many mischiefs.
   --Fuller.

   2. Cause of trouble or vexation; trouble.
   --Milton.

   The mischief was, these allies would never allow that the common enemy was subdued.
   --Swift.

   To be in mischief, to be doing harm or causing annoyance.

   To make mischief, to do mischief, especially by exciting quarrels.

   To play the mischief, to cause great harm; to throw into confusion. [Colloq.]

   Syn: Damage; harm; hurt; injury; detriment; evil; ill.

   Usage: Mischief, Damage, Harm. Damage is an injury which diminishes the value of a thing; harm is an injury which causes trouble or inconvenience; mischief is an injury which disturbs the order and consistency of things. We often suffer damage or harm from accident, but mischief always springs from perversity or folly.