We present a passively Q-switched, diode end-pumped, 1μm Nd:YAG laser with a single pulse energy in excess of 40mJ. To our knowledge, this is the highest single pulse energy reported for a passively Q-switched end-pumped laser. We achieved this with a novel pump scheme, which uses an engineered diffuser to create the necessary uniform gain distribution for efficient passive Q-switching. The system consists of a 3kW, 808nm, diode-laser stack pump source, and a set of collimating optics, with the engineered diffuser, to homogenise and couple the pump beam into the end of a 20mm diameter Nd:YAG laser rod. Q-switching is achieved with a Cr:YAG saturable absorber within a plane-parallel cavity. The 40mJ value was achieved despite a pump coupling efficiency of only 55%; hence we believe higher energies are achievable. The beam parameter product and pulse width were measured to be 12mm mRad and 18ns, respectively, which are consistent with those required for designation. We have investigated the pulse-to-pulse timing jitter of our system, which has been previously cited as the main drawback when implementing passive Q-switching for designation applications. We have achieved a reduction in timing jitter from 16 μs to 3.2 μs by environmental isolation of the laser resonator.