There are various spellings, but the most common is "bupkis", which seems to be Yiddish for "goat feces".
http://en.wiktionary.org/wiki/bupkis
I don't have SH4 installed currently, but if I'm remembering correctly the U-Boat add-on doesn't come with any single missions, which is why you aren't seeing any. There are some available, but I can't seem to remember where. Sorry.