Synonyms for Thickheaded:

adj.

ignorant (adjective)
dull, dumb, boneheaded, blockheaded, idiotic.
thick (adjective)
boneheaded.

n.

dopey, hebetudinous, ability, dumb, thick-witted, obtuse, doltish, dimwitted. stupid (noun)
dull, idiotic, ignorant.
thickheaded (noun)
boneheaded, loggerheaded, fatheaded, wooden-headed, thick-skulled, blockheaded, thick.

Other synonyms:

doltish, thick-witted, dimwitted. hebetudinous, dopey. obtuse. dumb. Other relevant words:
dopey, doltish, dull, hebetudinous, idiotic, obtuse, thick-witted, dimwitted, dumb, ignorant, ability.