The story
From the mathematical formula (∂𝒱)/(∂τ), the partial derivative of vega (𝒱) with respect to time (τ), pronounced as "D vega (by) D time".
Etymology from Wiktionary, CC BY-SA 4.0.
DvegaDtime
Today it means: A second-order measure of derivative price sensitivity, expressed as the rate of change of vega with respect to time, or equivalently the rate of change of theta with respect to changes in the volatility of the underlying asset. Full definition →