1928, "without appointment," from the verbal phrase, from walk (v.) + in (adv.). As a noun, meaning "walk-in closet," by 1946.
the emergency room was overrun with walk-ins
a deep walk-in refrigerator
walk-in closets
Wales
walk
walkabout
Walker
walkie-talkie
walk-in
walking
walk-on
walk-out
walk-over
walk-through