The French ancestor of amateur meant one who loves. It continues Latin amātor, lover or devotee, formed from amāre, love.
The slots select a Latin verb, its agent noun and a later English use, omitting the intervening French forms. Nonprofessional does not imply unskilled. The deeper origin of amāre is uncertain.
Source
Merriam-Webster: amateurWord History and Kids Etymology give the French meaning and Latin formation.