The Collaborative International Dictionary of English v.0.48:

Ribwort \Rib"wort`\, n. (Bot.)
   A species of plantain (Plantago lanceolata) with long,
   narrow, ribbed leaves; -- called also rib grass, ripple
   grass, ribwort plantain.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ribwort
    n 1: an Old World plantain with long narrow ribbed leaves widely
         established in temperate regions [syn: English plantain,
         narrow-leaved plantain, ribgrass, ribwort, ripple-
         grass, buckthorn, Plantago lanceolata]