chirk up

(idiomatic, intransitive) To become more cheerful, perk up.

Example: 2006, Anna Cypra Oliver, Assembling My Father: A Daughter's Detective Story, page 10
  She was terrified but in control. n. How, on the trip around the country my parents took the year after my birth, their mood sank so low that even my brother sensed it. "Chirk up, guys," he said. "Chirk up." They laughed at that.