To make a stupid, silly or childlike mistake in an email to someone. This could be a spelling or grammatical error, you could have addressed the email to one person and sent it t another or accidentally sent them the email before you were finished. Usually results in awkwardness and embarrassment.
Allie: Oh shoot, I just gusflubbered that email to my new boss and typed his name"Jeff" instead of "Geoff"
Fionna: I hate it when I gusflubber like that..hopefully he won't notice.
Allie: Still, it is embarrassing to gusflubber in an email to your new boss!