A lyricist is the person who writes the words of a song — not the tune, and not always the singer. In pop and musicals you often have a composer for the music and a lyricist for the words; sometimes one person does both, and then people may say singer-songwriter instead. You see the word in credits: lyrics by… It is more specific than poet. A poet may never write for music; a lyricist writes words meant to be sung. It is a normal music-industry word, not old-fashioned and not rude.