An output stream for message reporting. If set to nil, then no output will be produced. By default this is t.
The function break-on-unresolved-functions produces break-on-entry code for all currently undefined but referenced (that is, unresolved) foreign symbols, so that if an undefined foreign function is called from within the foreign code, a Lisp error will occur. Break-on-entry code will also be produced for any new unresolved symbols loaded later in your Lisp session.
The special variable foreign:*break-on-unresolved-functions* will, when set to non-nil, produce break-on-entry code for all new unresolved symbols that are loaded, but won't do so for symbols already loaded. By default this variable is set to nil.
LispWorks User Guide and Reference Manual - 20 Sep 2017