They undermine is the present indicative 3rd person plural of '' undermining '' whose meaning is decreasing somewhat, removing a part, shorten it, reduce it.
It is a synonym for damaged, deteriorating, decline.
3rd person plural of the present indicative form of the verb to undermine, ( less and held ) decrease in value, quantity or quality, damage, harm, downgrade, impair, dishonor, discredit, offend, discredit someone.