مَضْرَبٌ

is a verbal noun (Ham p. 129.) [See the sentence explaining the phrase ضَرَبَ فِى الأَرْضِ; and also the sentence next following it, towards the close of the first paragraph.]

And it is also a noun of place [and of time, like مَضْرِبٌ, which is the regular form]. (Ham ibid.) See the next paragraph, in five places.