Furthermore means in addition to what has already been said. Another point, and it supports the same case.
It is a formal connector and it belongs to essays, reports and letters of complaint, where it introduces a further argument. It sits at the start of a sentence with a comma after it. Moreover does the same job and sounds slightly less heavy, and what is more is the conversational version. Nobody says furthermore out loud without sounding as though they are reading.
1adverbfurthermoreIn addition to what has just been said.
“Furthermore, no one had been told about the change.”
Middle English
furthermore
English, today
furthermore
From Middle English furthermore, forthermore, forthermar (also as forther mo), equivalent to further + -more.
Etymology from Wiktionary, CC BY-SA