In Becoming (part 2), what do tu think Whistler means when he says, "Wrong, kid. tu got one más thing [to loose]," in response to Buffy stating that she can deal with everything because she has nothing left to loose.

Whistler means she still has herself to loose and what he means is answered when buffy is on the floor in the mansion and angelus is about to stab her with his sword and he says something like "no weapons, no friends...what's left" and buffy responds with "me"- confirming that she did have one más thing to loose and she realizes that now

I've always wondered about this too :\

I've always thought it had a dual meaning: that she had herself/her life to lose, and also giving up any chance of ángel returning.