This counts how many times the gateway's SynDefender feature timed out waiting for the final ACK of a TCP three-way handshake it had proxied on behalf of a protected server. Since SynDefender is specifically the gateway's SYN-flood countermeasure, a rising count here means the gateway is actively absorbing what looks like a SYN-flood attempt against a server behind it, rather than benign traffic. For example, this counter jumping from a handful per day to several thousand per minute during a denial-of-service attempt would prompt the admin to check asmSynatkCurrentMode for the gateway's active response.