praevenire

Latin praevenire combined prae (before) and venire (to come). To praevenire was to come before, to arrive ahead of, to anticipate. In classical Latin, the word was positive: to prevent something was to precede it, to be there first, to be ready. The English word prevent entered the language in the 15th century with this anticipatory meaning intact.

The King James Bible of 1611 uses prevent in its original Latin sense: 'Mine eyes prevent the night watches' (Psalm 119:148) means 'my eyes wake before the night watches' — I am up before dawn. 'Prevent us, O Lord, in all our doings' in the Book of Common Prayer means 'go before us, lead us, be there first.' Neither use involves stopping anything.

The meaning shift occurred because anticipating something and stopping it from happening are related actions. If you come before a problem, you also tend to stop it. By the 16th century, prevent had acquired the meaning of forestalling, and by the 17th century it meant blocking or stopping. The anticipatory sense faded, the obstructive sense dominated.

The old meaning surfaces in the legal phrase preventive detention — detaining someone before they commit a crime, anticipating their action — but most users of the word now experience it only as stop or block. The 'going before' is invisible.