the.com/killing frost

means A frost hard enough to kill tender plants outright, ending the growing season for anything not built to winter over.

from A plain compound from Old English 'cwellan' (to kill) and 'frost,' used descriptively by farmers and gardeners for centuries — this isn't a fancy coinage but a working phrase, the kind that shows up in almanacs and planting guides because growers needed a term for the frost that actually finishes the garden, as opposed to a light frost tender plants can shrug off.