Actually, נגב itself just means south. נגבהּ means "to the south", and is a relic of a rich system of gramattical cases which semitic-language nouns once had, it is the so-called "lative case" - an ending which means "to ...". We still use this lative case in very specific words like "הביתה" (to home), צפונה (to the north), etc. But not on general nouns (you cannot say "השולחנה" for "to the table" or "ישראלה" for "to Israel").