1
general::
Also, heart of the matter. The basic, central or critical point of an issue. For example, In this trial the bloodstains represent the crux of the matter, or We think the second clause is the heart of the matter. Although crux is Latin for "cross," in English it means "difficulty" or "puzzle," and it is from the latter that this expression is thought to be derived. The variant employs heart in the sense of "a vital part" (as it is in the body). The first term dates from the late 1800s, the variant from the early 1500s.
American Heritage Idioms
2
general::
Phrase(s): *the crux of the matter [and] *the root of the matter; *the heart of the matter
the central issue of the matter. (Crux is an old word meaning "cross." *Typically: be at ~; get at ~; go to ~; look at ~.) • All right, this is the crux of the matter. • It’s about time that we got to the heart of the matter.
McGrawhill's American Idioms And Phrasal Verbs