Dustoff! - That was what I was trying to remember. It started as a random callsign for a medical evacuation mission, was made permanent, and became associated with the requirement, the aircraft, the capability, etc that is still in use today. From a Dustoff website:
The DUSTOFF call sign was acquired by the 57th Medical Detachment (Helicopter Ambulance) in January, 1964, ironically and quite appropriately/proudly, by way of medical necessity.
The occasion was a bloody two-day operation, west of Saigon, near the Cambodian border in early Jan 1964 that churned out many casualties throughout a day and night of battle.
At the time, all combat units utilized randomly selected call signs controlled by the Navy Support Activity, Vietnam and were changed periodically in accordance with Signal Operating Instructions (SOI) in the interest of security. The call sign of 57th Med Det (HA) on that day and at that time, per the SOI, happened to be "DUSTOFF".
Fighting was heavy, casualties were mounting, evacs were continuous and communication between the evac helicopters and ground commanders was ever-running. At the time that the code changeover was ordered to occur in accordance the SOI, the major ground commander as well as the medevac aircrews became concerned that due to the battlefield chaos that included some loss of communications between units, switching to a new call sign may jeopardize evacuation. At the request of the ground commander, the 57th Med Det commander, MAJ Lloyd Spencer, agreed immediately to delay the call sign changeover while also requesting a temporary exception to the SOI from the approving headquarters. The exception was approved for a specified period (probably for 24 hours or possibly until the time of the next call sign change, per the SOI).
After the operation, MAJ Lloyd Spencer and MAJ Charles Kelly discussed the call sign dilemma and determined that a permanent aeromedical evacuation call sign would help avoid possible future mission impairment and more closely conform to the spirit of the Geneva Convention. (At the time, MAJ Spencer was outgoing commander - MAJ Kelly, incoming commander, assuming command on 11 Jan 64)
Within a few days, in concert with ground unit commanders, MAJ Spencer requested and received official approval for the permanent call sign change to "DUSTOFF".