Fig.: to exaggerate or say things that aren't true to make you seem better; to intentionally misinform or deceive someone

There's nothing true in what you are advertising to your potential customers. You're just blowing smoke, but sooner or later they'll find out and you'll be in serious trouble

Non c'è niente di vero in quello che stai pubblicizzando ai tuoi potenziali clienti. Stai solo vendendo fumo, ma prima o poi se ne accorgeranno e per te saranno guai seri

The politician sold hot air to his constituents, making big promises he knew he could not keep. Now he's been elected and he will have to account for his false promises

Il politico ha venduto fumo ai suoi elettori, facendo grandi promesse che non poteva mantenere, ma ora che è stato eletto siamo al redde rationem: dovrà rendere conto delle sue false promesse

The politician sold hot air to his constituents, making big promises he knew he could not keep. Now he's been elected and he will have to account for his false promises

Il politico ha venduto fumo ai suoi elettori, facendo grandi promesse che non poteva mantenere, ma ora che è stato eletto siamo al redde rationem: dovrà rendere conto delle sue false promesse

Usually, this idiom is related back to the magicians' world and shows. Stage magicians would release smoke to produce a distraction in order to perform their tricks unnoticed.
Over time, the phrase developed a figurative meaning. These days, it's used to describe the practice of misleading people by making untrue claims, exaggerations, or hollow promises.