Compound channels model situations where a sender and receiver both have only partial information about the stationary channel they are transmitting over. They can be used to analyse the robustness of a communication system. With this work we provide a coding theorem for a class of practically relevant quantum compound channel models, where we consider the system uncertainty as due to a stationary but unknown jamming signal. Namely, we give an explicit formula for the case of the lossy Bosonic channel in the presence of a semi-classical attack utilizing the injection of coherent states into the transmission line. Mathematically, this is modelled by letting transmitter and jammer access the two different ports of a beam-splitter, and the receiver observe one of the output ports. While the transmitter can modulate their input, the jammer must transmit a constant (but unknown) sequence of states. We conjecture a new quantum entropy power inequality and show its relation to the capacity under study.