WordNet sense 1 (impaired in skill by neglect):

out of practice

WordNet sense 2 (not having or showing or requiring special skill or proficiency):

unskilled

WordNet sense 3 (having accumulated rust):

rusted

WordNet sense 4 (ancient):

hoary

WordNet sense 5 (of the brown color of rust):

rust, rust-brown

WordNet sense 6 (being or having or characterized by hue):

chromatic