In some one's blog, it says," My cold is on its last legs." and means" it's almost finished."
I checked this phrase in the dictionary and it says" 人が死にかかって（(dying)、疲れきって(exhausted)、破産しそうになって(going bankrupt)."
Now I'm wondering why the sentence"My cold is on last legs" means "it's almost finished."
Another question is why"legS" not"leg". I know I should understand it as a phrase,though.
Do you use this phrase casually in your daily life?
Would you please show me some example sentences ?
Thanks in advance for your corrections and advice.