Verified answer

100倍; 100 times, times 100

返し; noun form of 返す, return

やで; end particle (colloquial)

Not sure if there are any similar expressions in English but what it means is "(get even by) returning 100 times of what one received." Normally something bad/negative he/she received like damage, insults, etc.

Edited: wording

It means like an eye for an eye.

100倍 is on hundred times, so 100倍返しやで would be "I'll get you back one hundred times over."