Dual-function radar-communication (DFRC) based on frequency hopping (FH) MIMO radar (FH-MIMO DFRC) achieves symbol rate much higher than radar pulse repetition frequency. Such DFRC, however, is prone to eavesdropping due to the spatially uniform illumination of FH-MIMO radar. How to enhance the physical layer security of FH-MIMO DFRC is vital yet unsolved. In this paper, we reveal the potential of using permutations of hopping frequencies to achieve secure and high-speed FH-MIMO DFRC. Detecting permutations at a communication user is challenging due to the dependence on spatial angle. We propose a series of baseband waveform processing methods which address the challenge specifically for the legitimate user (Bob) and meanwhile scrambles constellations almost omnidirectionally. We discover a deterministic sign rule from the signals processed by the proposed methods. Based on the rule, we develop accurate algorithms for information decoding at Bob. Confirmed by simulation, our design achieves substantially high physical layer security for FH-MIMO DFRC, improves decoding performance compared with existing designs and reduces mutual interference among radar targets.