Refers to da cheerful/casual whistling/humming/singing dat you're always super-glad/relieved to hear emanating from a mechanic, carpenter, or other repairman as he's working on your in-need-of-fixing item, since this unflustered tune-making indicates that whatever mending/alterations which are required to restore your device are merely "all in a day's work" or "old hat" with him; i.e., he's confidently proceeding with the task and is not concerned that there will be any snags. problems, or delays in said repairs.
Hearing the gloriously-welcome musical reassurance from a service-person produces a similar radiant hopefulness that you experience from being present during an "Ohhhhhhh...!" moment, since in both cases you feel pleasantly relieved that your device is indeed not irreparably damaged/worn/aged, and therefore it will probably be up-and-running again in a fairly short time.
by QuacksO June 12, 2019