at ease

1.  Also, at one's ease. Comfortable, relaxed, unembarrassed, as in I always feel at ease in my grandmother's house. The related idiom put at ease means "make comfortable, reassure," as in I was worried that the letter would not arrive in time, but the postmaster put me at ease. [1300s] For the antonym, see ill at ease.
2.  In a relaxed position in military ranks. The phrase is often used as a command for troops standing at attention to relax, as in At ease, squadron. The command stand at ease is slightly different. A British military dictionary of 1802 described it as standing with the right foot drawn back about six inches and one's weight put on it. An American version is to stand with one's feet slightly apart and the hands clasped behind one's back.

The adjective has 3 meanings:

Meaning #1: without strain or anxiety
  Synonym: relaxed

Meaning #2: free from worry or anxiety
  Synonym: easy

Meaning #3: socially at ease

The adverb at ease has one meaning:

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