  Terahertz (THz) communication is one of the most promising candidates to
accommodate high-speed mobile data services. This paper proposes a secure
hybrid automatic repeat request with incremental redundancy (HARQ-IR) aided THz
communication scheme, where the transmission secrecy is ensured by confusing
the eavesdropper with dummy messages. The connection and secrecy outage
probabilities are then derived in closed-form. Besides, the tail behaviour of
the connection outage probability in high signal-to-noise ratio (SNR) is
examined by carrying out the asymptotic analysis, and the upper bound of the
secrecy outage probability is obtained in a simple form by capitalizing on
large deviations. With these results, we take a step further to investigate the
secrecy long term average throughput (LTAT). By noticing that HARQ-IR not only
improves the reliability of the legitimate user, but also increases the
probability of being eavesdropped, a robust rate adaption policy is finally
proposed to maximize the LTAT while restricting the connection and secrecy
outage probabilities within satisfactory requirements.
