I looked up dollar in the dictionary and this is what I got for its origins as a word:

Low German daler, short for Joachimstaler, a coin from the silver mine of Joachimstal, now Jáchymov in the Czech Republic.

However, it doesn't explain how this word came to be used for the currency of Canada, US, and Australia (and probably smaller countries which weren't listed in that dictionary entry)? Also, why do we use dollars in Canada and not pounds (ditto for Australia)?