n Someone (or something) with thick skin. It is used for animals such as an elephant or a hippopotamus.
n (zoology) A member of the obsolete taxonomy | taxonomic group Pachydermata, grouping of thick-skinned, hoofed animals such as the rhinoceros, hippopotamus, elephant, pig and horse.
n (idiomatic) A person that is not affected by (does not care about) what others say about him or her.