"I do not know that word."
It’s literally “As for that word, [I] don’t know [it].” Basically you make “that word” the topic, the main thing that the sentence is talking about. It’s pretty common to see this in Japanese negative clauses:
- この本はまだ読みませんでした。 “As for this book, [I] haven’t read [it] yet.” => “I haven’t read this book yet.”
- バスには乗りません。 “As for the bus, [I] don’t take [it].” => “I don’t take the bus.”
I assume the reason for this is that if there was no explicit topic, the implied “I” would be understood to be the topic. That is something you don’t want in most contexts because it would be equivalent to emphasising the “I” in English:
- [私は]この言葉を知りません。 “As for me, [I] don’t know that word.” => “I don’t know that word [but that other person does].”
- [私は]この本をまだ飲みませんでした。 “As for me, [I] haven’t read this book yet.” => “I haven’t read that book yet [but my friend has].”
- [私は]バスに乗りません。 “As for me, [I] don’t take the bus.” => “I don’t take the bus [but my daughter does].”