Etymonline avers the etymology for 'exert' to be from:
1660s, "thrust forth, push out," from Latin exertus/exsertus, past participle of exerere/exserere
"thrust out, put forth,"from ex- "out" (see ex-) + serere "attach, join" (see series). Meaning "put into use" is 1680s.
ex- + serere literally signifies 'to attach or join out', which doesn't make sense. How did they compound to signify
"thrust out, put forth"? How ought this compound be understood?