In southern India it's common for the last name to simply be the father's name (but I do know one guy whose last name is his mother's name).

Like, my name could be "Foo", my father's name could be "Baz" so the given name field of the passport could be "Foo Baz", and the last name would be blank.

But the visa form for certain countries don't accept that, so they put in "LNU".

Anyway, these days people are more aware, so when applying for passport they specify the given name as "Foo", and the surname as "Baz" (parent's name), even though that's really not the surname.