On most IRC clients, doing
/whois on a nick will display their idle time. However, irssi does not show this. Is there any way to make it show the idle time?
This depends on the IRC server. Normally, the WHOIS query would be processed by the server you are on. But on IRC, servers only know idle time and away status for directly connected users. (This is to avoid a lot of unnecessary traffic between servers.)
To see idle time, put the nickname twice, like this:
This is aliased to /WII by default:
It is also in the builtin help:
This will cause the query to be forwarded to the target user's server, which knows the idle time.
Note that on some networks, cross-server WHOIS queries can be heavily throttled (example: freenode).