It is used to describe or vent frustration (although often minimal and humorous) toward someone who has not used an Internet search engine before asking a question, often one that when type directly into the search engine will provide the answer.
The etymology comes from the idea of there being a device built into computers that can be activated by a remote user where a cartoonishly large mallet comes out of the side of the computer and comes crashing down toward the non-search engine user's forehead. Google's multicolored logo is on the bottom side of the mallet and thus it will be the last thing they see before being struck, hopefully beating the idea into their subconscious.
> Where was Barack Obama born?
The guy worked at Google. I had to googlemallet him extra hard.