1
general::
Also, breathe easily or freely. Relax, feel relieved from anxiety, stress, or tension. For example, Now that exams are over with, I can breathe easy, or Whenever I'm back in the mountains, I can breathe freely again. This idiom originally (late 1500s) was put as breathe again, implying that one had stopped breathing (or held one's breath) while feeling anxious or nervous. Shakespeare had it in King John (4:2): "Now I breathe again aloft the flood." The variant dates from the first half of the 1800s.
American Heritage Idioms
2
general::
Phrase(s): breathe easy
to assume a relaxed state after a stressful period. • After this crisis is over, I’ll be able to breathe easy again. • He won’t be able to breathe easy until he pays off his debts.
McGrawhill's American Idioms And Phrasal Verbs