A Bunny Suit is what people have come to call
cleanroom coveralls with boots and a hood. The term's
progeniture is one of those few instances where the electronic industry waxes
whimsical. (FWIW I think the term "
Bluetooth" is another example.)
Most point to Intel's Bunny Ad in the mid '90's as the source for the name. However, the term was being used by Intel as far back as the 80's, and they were wearing them since they became standard Intel cleanroom attire.
The reason it was called a Bunny Suit by the techs forced to wear them is pretty obvious to anyone who has seen A Christmas Story. The only booted, hooded, costume most of the people knew from their childhood were pajamas, usually representing the cutest of pests, the rabbit. There were other animals represented by bootie pajamas, and maybe if the film version of Where the Wild Things Are had been made prior to 1980, we would be calling cleanroom coveralls "Fox Suits" now.