What is the expiation for one who swore to abandon masturbation then returned to it, and is this oath considered a "yamin ghamus" (a false oath taken knowingly)?

Masturbation is forbidden and has severe harms. One must repent from it and seek Allah's help. As for breaking an oath not to return to it, you are obligated to offer expiation for each oath you broke, according to the majority of scholars. There is another opinion that it is a single expiation if one had not expiated before. The first opinion is more correct and safer. This oath is not considered a yamin ghamus (a false oath taken intentionally to deceive).