When did they start using the word "Scrubbed" for launches?

Many speculation exists for why the term 'scrubbed' is used in relation to launches. One of these is that during world war II, when the Air force planned bombing raids. These  Air force missions had their briefings done with illustration of the details of the launch and and flight drawn on chalkboards. When some of mission flight takeoffs were canceled (due to bad weather or a malfunction or other reasons) the details were then scrubbed off of the chalkboard. This idea of the mission being scrubbed meaning that it is no longer to be followed through with, proceeded to this modern day rocket and aviation launches.

tube or goog will be great resources to find the answer to the question that you're asking

it is very well related to a lot of things but with my limited knowledge i can confidently say that the answer is space.