Desistir means 'to give up' or 'to desist'. It's a strong verb that conveys a sense of resignation or forced surrender.
In the context of this song, the title itself, "Desisto (Obrigado a desistir)" (I Give Up (Forced to Give Up)), highlights the central theme. The singer is compelled to give up on a love, even though he doesn't want to, making this word particularly poignant and central to the song's emotional core.