Grey area an area of law or morality which does not fall into any predefined category and which is a matter of uncertainty. In the 1960s, grey areas in British planning parlance referred to places that were not in as desperate a state as slums but which were in decline and in need of rebuilding.
Grey Friar a Franciscan friar, so named because of the colour of the order's habit.
grey goo a mass of self-replicating nanoscale machines proliferating uncontrollably and destroying or damaging the biosphere, postulated as a danger of the use of nanotechnology.
the grey mare is the better horse the wife rules, or is more competent than, the husband; the grey mare can be used allusively for a woman who is the dominant partner in a marriage. The saying is recorded from the mid 16th century, but a similar idea is found a little earlier in More's Dialogue of Images (1529), ‘Here were we fallen into a great question of the law, whether the grey mare be the better horsse…or whether he have a wise face or not that looketh as like a fool as an ewe looketh like a sheep.’
grey matter the darker tissue of the brain and spinal cord, consisting mainly of nerve cell bodies and branching dendrites; informally, intelligence. The phrase little grey cells was used by Agatha Christie's detective Hercule Poirot to describe his intelligence.
See also all cats are grey in the dark at cat, little grey cells, men in (grey) suits.