On his legs. Mr. So-and-So is on his legs, has risen to make
On its last legs.
Moribund; obsolete; ready to fall out of cognisance. To set on
his legs. So to provide for one that he is able to earn his living
without further help. To stand on one's own legs. To be
independent: to be earning one's own living. Of course, the allusion is
to being nursed, and standing “alone.” (See Bottom.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894