WordNet sense 1 (armor carried on the arm to intercept blows):

buckler

WordNet sense 2 (hold back a thought or feeling about):

harbor, harbour

WordNet sense 3 (hard outer covering or case of certain organisms such as arthropods and turtles):

carapace, cuticle, shell

WordNet sense 4 (protect, hide, or conceal from danger or harm):

screen