Grex

I remember that, several years ago, I tried to memorize the meaning of an English word gregarious (“sociable”), but somehow I couldn’t remember it, even though I tried to repeatedly. Then, I looked it up on the Internet, and understood this word by associating it with another English word segregate, which I had already known. The -greg- part in English often means “flock”, which comes from the Latin grex meaning “flock”. The word segregate can be interpreted as “separate a flock”, because the prefix se- indicates the sense “apart” (cf. separate), and it was totally understandable.

There are some more with this substring greg: aggregate, congregate, and egregious. The first one aggregate is used as a noun, adjective, and verb, and they have the senses related to “combining into a single group”. The second one congregate means “to gather into a crowd”. Egregious is a bit different and means “conspicuously bad” (from the original meaning “standing out from the flock”)¹.