Definition of "unclimbable"

The base of each [floodlight] tower forms an outdoor-type sub-station compound with a 7ft unclimbable fence which houses the extra high-voltage ring main switchgear, the transformer and the medium voltage switchgear.

1963 August, “The semi-automatic Tees Yard of the North Eastern Region”, in Modern Railways, pages 123–124

Yet every world should have at least one unclimbable mountain.

1970, Larry Niven, Ringworld, page 118