What does [UNREPLIED] mean?

[Steve Snyder]

I've had this same entry in my connection tracking table (seen via  "cat 
/proc/net/ip_conntrack") for about a week.

tcp      6 175631 ESTABLISHED src=192.168.0.3 dst=151.193.160.75 
sport=1079 dport=443 [UNREPLIED] src=151.193.160.75 dst=12.222.5.178 
sport=443 dport=1079 use=1

Can anyone tell me what the [UNREPLIED] means and why it should linger so 
long in the table?

Thanks.