"When incumbent was first used in English in the 15th century, it referred to someone who occupied a benefice—a paid position in a church. This was often a lifetime appointment; the person could only be forced to leave the office in the case of certain specific legal conflicts. In the mid-17th century, incumbent came to refer to anyone holding any office, including elected positions. These days, in the American political system, incumbent generally refers to someone who is the current holder of a position during an election to fill that position. Incumbent came to English through Anglo-French and derives from the Latin incumbere, meaning "to lie down on.""
https://www.merriam-webster.com/dictionary/incumbent
https://en.wikipedia.org/wiki/List_of_presidents_of_the_United_States