The entry for "dust" from LDOCE says:
→ HOUSEHOLDdry powder consisting of extremely small bits of dirt that is in buildings on furniture, floors, etc. if they are not kept clean:
All the furniture was covered in dust.
a dust (BrE)
→ CLEANINGthe act of dusting something:
I need to give the sitting room a dust.
→ CLEANINGto clean the dust from a surface by moving something such as a soft cloth across it:
Rachel dusted the books and the bookshelves.
Now, the entry for "undust" from Wiktionary says:
- (obsolete) To free from dust.
I do notice the entry being marked as "obsolete" for undust, and also I remember my English classes as a child where we had a rectangular cubic sponge called a "duster" that we used to dust/undust the chalk on the blackboard, however every time I'm cleaning my desk, computer, furniture, etc. I'll automatically think of "undusting" and then will force myself to undo and say "dusting" instead, just to convince myself that the latter is the grammatical one; however I can't see the logic behind this verb and its usage based on general English grammar.
Google Ngrams also shows up some results (even if not many) for undust:
Is undust incorrect? Did it exist as a correct verb years ago, and can I still use it?
I feel paranoid to say: "I'm dusting my keyboard!" because it makes me feel that it has the opposite meaning, as if I'm spreading dust and particles on my keyboard, especially since these chores aren't all that commonly spoken with today's lifestyle, and stating the former sentence might sound a bit odd.
P.S. Note that "dust something off" is slightly different in usage e.g. "They were dusting off leaves and twigs.", and based on my understanding from the definitions in dictionaries, saying "
I'm dusting my keyboard off." isn't correct.