Flagelarnos comes from the verb flagelar, meaning 'to flagellate' or 'to whip'. It's a strong and somewhat dramatic word that isn't commonly found in everyday conversation or many song lyrics.
In "Nos Vamos Juntos," the phrase "De flagelarnos para querernos" (To stop flagellating ourselves in order to love each other) uses this word metaphorically. It suggests a deep emotional or psychological self-punishment within a relationship, implying that the couple needs to stop hurting themselves or each other in order to truly love. This powerful imagery makes it a very memorable and thought-provoking word.