an instant: the split second in which something breaks in with no warning at all
This is a noun, not an adverb. It names the split second itself, the blink of time in which a thing arrives, and Hebrew uses it much the way English uses the phrase "in an instant." Sometimes it stands on its own, and sometimes it follows a short preposition, but the effect is the same: whatever happened, happened in no time at all and with nobody braced for it.
Six of its seven appearances describe disaster, and the picture is remarkably consistent. The ruin is not merely fast; it is unforeseen, and there is no interval in which to react.
- a stubborn man is broken in an instant, past all mending (Proverbs 29:1)
- a bulging wall gives way and shatters in an instant (Isaiah 30:13)
- creditors a debtor never counted on rise up against him (Habakkuk 2:7)
- a swarming army is blown away like fine dust (Isaiah 29:5)
Three times the word stands right beside its close relative, the adverb פִּתְאֹם, in a doubled phrase usually rendered "in an instant, suddenly" (Numbers 6:9; Isaiah 29:5; Isaiah 30:13). Stacking two words that mean nearly the same thing is a Hebrew way of turning up the volume: not just quickly, but with no warning whatsoever.
The seventh use is different, and it is legal. In the law of accidental killing the word describes the killer rather than the victim: he shoved the man on the spot, with no grudge behind it and no ambush laid, which is exactly what keeps the death out of the murder category (Numbers 35:22). Here the word points to an absence of forethought, and the English "suddenly" carries only half of that.