The unglossed headword is imperfect indicative active, third person singular, of the verb r(ai/nw, which has the meanings given here (pertaining to either liquids or solids: see LSJ entry at web address 1). It must be quoted from somewhere but is not attested in this form elsewhere. (Adler notes Bernhardy's emendation to e)/rrainen, as in e.g. Josephus, Jewish Antiquities 3.205.)