Princeton's WordNet

1. walk-in(noun)

   person who walks in without having an appointment

   "the emergency room was overrun with walk-ins"

2. walk-in(noun)

   an operative who initiates his own defection (usually to a hostile country) for political asylum

3. walk-in, waltz(noun)

   an assured victory (especially in an election)

4. walk-in(adj)

   a small room large enough to admit entrance

5. walk-in(a)(verb)

   (of e.g. closets or refrigerators) extending very far enough back to allow a person to enter

   "a deep walk-in refrigerator"; "walk-in closets"

6. walk in(verb)

   enter by walking

   "She walks in at all hours, as if she lived here"