Other forms: exerted, exerts, exerting
To exert is to apply force, effort or influence. Putting real weight behind something.
The commonest use is exert influence or exert pressure, where nothing physical is happening at all. The physical sense usually comes back as exert yourself, meaning to make a real effort, and it very often appears in the negative advice not to exert yourself. The noun exertion means hard physical effort, and it is the word doctors use when telling somebody to avoid it. Note that you exert force on something, never at it.
To exert is to thrust out. Latin exserō joins ex, out, to serō, to bind or join, so exerting force is pushing it out of yourself.
Proto-Indo-European
*ser- (bind)
Latin
exsertus
English, today
exert
Built from
From Latin exsertus, past participle of exserō (“to stretch or thrust forth or out”), from ex- + serō.
Etymology from Wiktionary, CC BY-SA