Malice aforethought is the "premeditation" or "predetermination" (with malice) required as an element of some crimes in some jurisdictions and a unique element for first-degree or aggravated murder in a few. Insofar as the term is still in use, it has a technical meaning that has changed substantially over time.
Etymology
Malice aforethought is a direct translation of the Law French term malice prépensée, so the adjective follows the noun as in French.