Unknown Word Error by Neil Cerutti begins here.

"Provides Infocom-style parser messages such as 'I don't know the word 'kludge'.'"

Include (-
[ FindUnknownWordToken wordnum
w twn numwds;
#ifdef TARGET_GLULX;
numwds = parse-->0;
#ifnot;
numwds = parse->1;
#endif; ! TARGET_GLULX;
! Throw out invalid word numbers
if (wordnum <= 0 || wordnum > numwds) rfalse;
twn=wn; ! save the value of wn so it can be restored
wn=wordnum;
while (1)
{ w=NextWordStopped();
if (w == -1) { wn=twn; rfalse; }
if (w == 0 && TryNumber(wn-1) == -1000)
{ wordnum=wn-1;
wn=twn;
return wordnum;
}
}
];
-)

Include (-
[ PrintToken wordnum
k l m numwds;
#ifdef TARGET_GLULX;
numwds = parse-->0;
#ifnot;
numwds = parse->1;
#endif; ! TARGET_GLULX;
if (wordnum <= 0 || wordnum > numwds) return;
k=WordAddress(wordnum);
l=WordLength(wordnum);
for (m=0: m < l: m++)
print (char) k->m;
];
-)

Include (-
[ DontKnowError
wordnum;
wordnum=FindUnknownWordToken(2);
if (wordnum ~= 0)
{ print "I don't know the word ~";
PrintToken(wordnum);
"~.";
}
else rfalse;
];
-)

To say dunno:
    (-
return DontKnowError();
-)

Rule for printing a parser error when the parser error is can't see any such thing (this is the don't know that word rule):
    say dunno instead.

Unknown Word Error ends here.