We analyze the secrecy outage probability of a dual-hop radio frequency (RF)-free space optical (FSO) transmission systems. We assume that the eavesdropping occurs at the RF channel which experiences Nakagami-m fading while the FSO link suffers from Malaga turbulence-induced fading. More precisely, we derive an analytical expression for the lower bound of secrecy outage probability (SOP) to study the secrecy performance of dual-hop RF-FSO transmission system with the fixed-gain amplify-and-forward relaying protocol.