A person who adheres to egoism, a philosophical or ethical stance that prioritizes self-interest. This is a formal, academic term.
As an egoist, he holds an ethical view that prioritizes his own interests above all.
利己主義者 is a more common, everyday term for a selfish person, while 主我主義者 is a formal philosophical term for an egoist.
Compound of 主我 (shuga, 'ego' or 'self as primary') + 主義 (shugi, '-ism') + 者 (sha, 'person'). The exact historical derivation is uncertain; the term is used in philosophical contexts.