at rest

1.  In a state of inactivity or repose, either physical or mental. For example, The doctor's clear explanation put her mind at rest. Chaucer used this idiom in Troilus and Cressida (c. 1374): "I mine heart set at rest upon this point." Also see lay at rest.
2.  Dead, as in His soul is now at rest with his forebears. This usage, employing rest to refer to death's repose, is less common today. [1300s]

The adjective has 3 meanings:

Meaning #1: not in physical motion
  Synonyms: inactive, motionless, static, still

Meaning #2: in a state of repose or especially sleep

Meaning #3: (euphemistic) "he is deceased"
  Synonyms: asleep, at peace, deceased, departed, gone

The adverb at rest has one meaning:

Meaning #1: in a relaxed position, of soldiers
  Synonym: at ease