idiom/adjective phrase: to be mentally deficient in a very noticeable manner.
Etymology:
From the fact that many cases of mental retardation involves having an abnormal number of chromosomes.
Notes:
Funnily enough, means the same thing as "to have a couple
extra chromosomes". "To be short a couple chromosomes" is perhaps a more natural way of expressing the idea as it plays to the
human intuition that "more is better" and also because it is very easy to imagine how someone without the
normal number of chromosomes could turn out
retarded; however, "to have a couple
extra chromosomes" may actually be more accurate in most cases. The most common genetic cause for mental retardation is Down's syndrome, which actually involves having more chromosomes than the normal person (a third copy of chromosome 21, to be precise).
