Francophonie· hard· old-fashioned
sentir le sapin
“to smell of fir”
Meaning
To seem close to death
Explanation
Said of anyone who seems to be on their last legs. The smell in question is a coffin's.
Example
Depuis son opération, le vieux monsieur sent le sapin, selon ses voisins.
Since his operation, the neighbours say the old man looks like he's not long for this world.
Origin
It plays on fir being the wood coffins were traditionally made from, so smelling of it became a grim way of saying someone looks close to the end.
Source