Usage: WHOWAS <nick>
For a certain amount of time after a client has left the server you can still get information about that client. The information is the same as the information given by the WHOIS command.
If you just missed someone, but you want to know the whois info and forgot to check, you can WHOWAS them. You can also use WHOWAS if someone keeps switching nicks and you cannot whois them easily.