To make six exact copies of something.
This word is very rare and technical, and it's usually used in very specific situations, like in science or technology.
Examples:
I will sextuplicate this report for everyone.
The machine can sextuplicate the key.
She sextuplicated the important document for the meeting.
I don't think we need to sextuplicate the recipe, one copy is enough.
The scientist was able to sextuplicate the DNA for the experiment.
He sextuplicated the map for all of us to use.