As a legal term mental deficiency can imply many things. The legal definition of any given mental condition is what the doctor sys it is. So it could be whatever the best expert says it is. As the question is stated, the answer is you can call anybody anything you want pretty much in America and it is not illegal.

The only way in which calling someone mentally deficient would be illegal would be if it is considered slander, and met all of the following requirements: a) it must be false (according to a legal or medical definition of mentally deficient, you prove that the person is not mentally deficient) b) the statement must negatively affects the person's reputation (the person must show that they were actually harmed by the statement) c) the maker of the statement must have had malice (he or she knew it was false and meant to do harm) d) at least one other person must hear it (or read it, if it was written) It might be argued that calling someone mentally deficient would be considered a \"hate crime\" under the Violent Crime Control and Law Enforcement Act (H.R. 3355, 1994) which amended earlier hate crimes legislation to include disabilities. If mental deficiency is recognized as a disability, then it would be a hate crime. However, hate crimes aren't \"crimes\", per se. Hate crimes, at least in the United States, are simply things that make the sentence harsher for other crimes. So using hate speech may not be a crime (depending on wheather or not it is slander, see above), but using hate speech while committing an assault would increase the sentence if convicted of assault.