Query
From Scriptwiki
Opens a query window to the specified nickname. If a message is provided, it is sent.
/query [-n] <nick> [message]
If the -n switch is specified, the window is opened in a minimized state.
Example
/query Dana
Opens a query window for Dana.
/query TargetNick You suck!
Opens a query window and sends message You suck! to TargetNick.