Other forms: enlisted, enlisting, enlists
To enlist is to join the armed forces of your own accord. Signing the papers rather than being called up.
The volunteering is the point, and it stands in contrast to being conscripted or drafted. It takes in: enlist in the navy, enlist in the army. There is a very common second sense with nothing military about it, since enlisting somebody's help means persuading them to join in, and enlisting support is the standard phrase in campaigns. The noun for a person who has enlisted is an enlisted man, in American usage.
Proto-Indo-European
*leys- — to trace, track
Proto-West Germanic
*līstā
Medieval Latin
lista
Old French
liste
Old English
līste — hem, edge, strip
Middle English
list
English
list
English, today
enlist
Built from
From en- + list. list comes from Middle English list, liste (“band, stripe; hem, selvage; border, edge, rim; list, specification; barriers enclosing area for jousting, etc.”), from Old English līste (“hem, edge, strip”), or Old French liste, listre (“border; band; strip of paper; list”), or Medieval Latin lista, all from Proto-West Germanic līstā, possibly from Proto-Indo-European leys- (“to trace, track”). 2
Etymology from Wiktionary, CC BY-SA