What is the fifth coloum in the output of "ip -stat neighbour show" stand for?
After a look at iproute2 source code, the fifth fields gives timer information on ARP cache entries:
X/./.: Number of seconds since theARPentry was last used./X/.: Number of seconds since theARPentry was last confirmed././X: Number of seconds since theARPentry was last updated
Those timers are notably used to manage stale ARP entries and decide when a new ARP request should be issued. Refer to this insightful answer for more info about ARP age timeout.