irssi/docs/help/in/completion.in

@SYNTAX:completion@

Irssi can "complete" some words for you - you can write just first few letters
of the word and press TAB (or any other key with action word_completion
assigned). Then, irssi will choose the most probable matching word and inserts
it to the command line. You can press TAB repetitively and irssi will replace
the chosen word with another possible words matching to the letters you wrote.

The words chosen usually depend on their context - they can be filenames or
command names, but most frequently they are nicks of people on same channels as
you are. However, you can have global list of own completions, which apply to
all contexts. This command is dedicated to maintaining of such a list.

Without any parameters, /COMPLETION displays list of all user completions.

/COMPLETION [-auto] <prefix> <string> adds completion which gets triggered when
you press TAB (or any other key with action word_completion assigned) after
writing <prefix> specified and which expands to <string>. When -auto is
specified, the completion gets triggered even when you press SPACE or ENTER (or
any other key with action check_replaces assigned) after the <prefix>. If
there's already some <prefix> completion in the list, it will get replaced.

/COMPLETION -delete <prefix> removes completion of <prefix> from the user
completion list.

Note: When -auto is specified for the <prefix> you want to remove you have to 
quote it like /COMPLETION -delete '<prefix>'. Works with doublequotes aswell.

See also: BIND