We report on a fossil stellar merger for the subgiant primary of the visual binary HR 3750. The subgiant leads to an age T similar or equal to 3.2 Gyr for a mass M-A = 1.39 +/- 0.09 M-circle dot, in contradiction with its low iron-to-magnesium abundance that classifies it as a Population II (thick-disk) star. Upon the assumption of an ancient source, and since there appears to be no inner Aa Ab subsystem for HR 3750, the mass of the subgiant primary can only be understood in terms of a merger with a former tertiary component. In a mass conserving scenario, and with M-Aa = 1.03 0.03 M-circle dot as the likely progenitor mass of the primary, the mass of the accreted companion is suggestive of an M dwarf at M-Ab = 0.36 0.03 M-circle dot.