snift
A deliberate release of pressure and vapor, such as from a steam engine, or bottling equipment.
The proper use of the air cock is, that after the engine is got steadily to work, and the snift emits no more than it should do, that the air cock should be opened as much as it can be to let the engine come fairly in, that is, for the piston to come fully down, and if it goes out in preference to that of coming in, the better.
Where it comes from
Imitative. Compare Middle English snifteren (“to sniffle”).
Adapted from the Wiktionary entry snift (CC BY-SA 4.0).