À propos de bottes = With reference to nothing in particular; With no reference to the subject in hand.
Il le dit à tout propos = He says it on every occasion, at every turn.
Il l’a fait de propos délibéré = He did it of set purpose; He had made up his mind to do it.
Il l’a fait fort mal à propos = He did it very unseasonably, just at the wrong time.
Propre
C’est du propre (ironic.) = A fine thing indeed.
Il n’a rien en propre = He has nothing of his own.
Un propre-à-rien = A good-for-naught.
Propre à tout et bon à rien = Jack of all trades and master of none.
Propre comme un sou neuf = As clean as a whistle; As neat as a new pin.