erc-fill-function

erc-fill-function is a variable defined in `erc-fill.el'.
Its value is

erc-fill-variable

• This variable may be risky if used as a file-local variable.

Documentation:
Function to use for filling messages.

Variable Filling with an `erc-fill-prefix' of nil:

this is a very very very long message with no
meaning at all

Variable Filling with an `erc-fill-prefix' of four spaces:

this is a very very very long message with no
meaning at all

Static Filling with `erc-fill-static-center' of 27:

foo bar baz
foo bar baz quuuuux
this is a very very very long message with no
meaning at all

These two styles are implemented using `erc-fill-variable' and
`erc-fill-static'. You can, of course, define your own filling
function. Narrowing to the region in question is in effect while your
function is called.

You can customize this variable.