The Collaborative International Dictionary of English v.0.48:

doormat \doormat\ n.
   1. a mat placed outside and exterior door for wiping the
      shoes before entering. [WordNet sense 2]

   Syn: welcome mat.
        [WordNet 1.5]

   2. (figurative) A person who is habitually abused, taken
      advantage of or humiliated; sometimes, one who is
      physically weak; as, they used him for a doormat. [WordNet
      sense 1]

   Syn: weakling.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

doormat
    n 1: a person who is physically weak and ineffectual [syn:
         weakling, doormat, wuss]
    2: a mat placed outside an exterior door for wiping the shoes
       before entering [syn: doormat, welcome mat]