hardening off
n.— «It is best not to put seedling so young directly out in the garden. “Hardening off” is the term for gradually acclimating them to outdoor conditions. I use an half-way house, also known in garden circles as a cold frame, i.e., an enclosed, bottomless box with a clear glass or plastic lid that can be propped open for ventilation or closed against a chilly night.» —“Gardener to tomatoes: Step outside!” by Valerie Sudol Star-Ledger (New Jersey) May 6, 2008. (source: Double-Tongued Dictionary)
Further reading
A librarian opens a book and finds a mysterious invitation scribbled on the back of a business card. Another discovers a child’s letter to the Tooth Fairy, tucked into a book decades ago. What stories are left untold by these forgotten...
Sean in Oneonta, New York, says that when he was growing up in New Jersey, his family would pile in the car and set off on a surprise adventure, whether a short distance or long, and the kids would be told only that they were going on Buxtehude...
This is not a new term. I first heard and read it in the early 1980’s and it is probably older than that.
This meaning is found in the Collins online dictionary.
http://dictionary.reverso.net/english-definitions/harden%20off