Rich hidden unconventional orders with pseudogap formation, such as the intersite bond order (BO), attract increasing attention in condensed matter physics. Here, we investigate the hidden order formation in organic unconventional superconductor κ-(BEDT-TTF)₂X. We predict the formation of d-wave BO at wavelength q=Q_B=(δ,δ) (δ=0.38π) for the first time, based on both the functional renormalization group (fRG) and the density-wave equation theories. The origin of the BO is the quantum interference among antiferromagnetic spin fluctuations. This prediction leads to distinct pseudogap-like reduction in the NMR 1/T₁ relaxation rate and in the density-of-states, consistently with essential experimental reports. The present theory would be applicable for other strongly correlated metals with pseudogap formation.

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