How do I hide my username in irc?

To change your username in irssi do:

/set user_name <whatever>

From within the client.

A common technique for hiding the host name of a client is known as "irc bouncing". It involves getting a shell on a public server and connecting to an IRC network through that.

The parameter you want to change in your question is the user name. It's probably based on the email convention "user at host" aka [email protected], for example. The username can usually be changed in the IRC client.

In irssi, the command is /set user_name [parameter]

E.G.:

prompt>>> /set user_name jaro


08:26 [server]
08:26 user_name = jaro

prompt>>> /connect irc.efnet.org

08:26 >  ! Irssi: Looking up irc.efnet.org
08:26 >  ! Irssi: Connecting to irc.efnet.org [208.51.40.2] port 6667
08:26 >  ! Irssi: Connection to irc.efnet.org established
(...)
08:26 >  ! Irssi: Your nick is owned by jaroslav  ([email protected])

prompt>>> /whois rakhmato

08:26 >  ! rakhmato ([email protected])
08:26 >  !  ircname  : jaroslav
08:26 >  !  server   : irc.eversible.com (Eversible.com Internet Services)
08:26 >  !  hostname : 84.202.44.54
08:26 >  !  idle     : 0 days 0 hours 0 mins 21 secs (signon: Wed Mar 13 08:22:23 2013)
08:26 >  ! End of WHOIS