to sweep or scrape away to destruction; and so, in the passive, to be swept away and perish
The picture underneath this word is physical: scraping or sweeping something off a surface until it is gone. A razor sweeps the hair clean off a face (Isaiah 7:20), and from that concrete image the word reaches its common use, removing people or things by destroying them.
Most often it describes disaster falling on the guilty and the danger that the innocent will be caught up in the same sweep. When Abraham pleads over Sodom he asks whether God will sweep away the righteous along with the wicked (Genesis 18:23), and the angels warn Lot not to be swept away in the city's punishment (Genesis 19:15). The same fear runs through the warning to stand clear of Korah's tents (Numbers 16:26).
In its passive uses the word turns to the victim's side: to be swept away is simply to perish, to be finished off. Saul's men will perish in battle (1 Samuel 26:10), David fears he will one day be swept away by Saul's hand (1 Samuel 27:1), and even the beasts and birds are said to be swept off the land under judgment (Jeremiah 12:4).
Hebrew has other words for ruin, but where שָׁחַת pictures spoiling a thing until it is marred, this word pictures removal, scraping something clean off the map.