RP
It suggests that one has made her/his last effort before her/his demise.
busterwasmycat
not that I have ever heard that one, but I assume that it would be talking about the desperate efforts of a person or group when they are destined to fail but haven't quite given up yet. They might be beaten but they can still cause some harm if you aren't careful.
Anonymous
Beware the last kicks of a dying horse, for when the animal knows it is dying it gets a surge of adrenaline from the fear, and the last kicks will be the strongest and most dangerous. It isn't so much a metaphore as it is a proverb.