I only know of one Koby and it's my friend's son so to me I think boy. However, it does make a great girls name.

The meaning is ordinary though. I'll check my baby names book shortly and see what it says.

ETA: it's only in the boys section in my book and neither versions have meanings. Jacob, however, has the meaning you mentioned earlier so I guess that's why. Jacob was (is?) on our list but I hadn't considered the meaning (yet) but I'm not fond of it either!

Koby - Polish, diminutive of Jacob

Coby - English, diminutive of Coburn or Jacob