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