What are "Bukah u'Mevukah u'Mevulakah"?
Rashi: They are, respectively, empty, emptied via Bokekim (plunderers), and its walls were breached.
Radak: Bukah and Mevukah are expressions of empty; the matter is repeated to strengthen it. Mevukah has an added Mem, like "Uzi u'Ma'uzi" (Yirmeyahu 15:19). Ninveh will be empty of all good, and Mevulakah (cut off).
Malbim: Bukah and Mevukah are expressions of empty; Mevulakah - all is destroyed.
What do we learn from "v'Lev Namas"?
Radak: The hearts of the residents will melt [Malbim - amidst great fear].
What is "u'Fik Birkayim"?
Rashi: The knees stumble [Malbim - when they flee]. This is like "l'Fukah ul'Michshol" (Shmuel I, 25:31). Radak - when one is very afraid and panics, his knees knock. Targum Yonasan is like this. And so it says about Belshatzar "v'Archubaseh Da l'Da Nakshan" (Daniel 5:6).
Why do the loins tremble?
Malbim: It is amidst great breakage and sadness.
What is the meaning of "Kibetzu Farur"?
Rashi: Black like a pot.
Refer to Yo'el 2:6:3:2-3 and the note there.