Douglas Harper's Etymology Dictionary
1894 in biological sense "repetition of parts in living things;" earlier in rhetoric, "synecdoche in which totality is expressed by contrasting parts" (such as high and low, young and old); from Modern Latin merismus, from Greek merismos "a dividing, division, a partition," from merizein "to divide," from meros "part, share" (see merit (n.)). Related: Merismatic.
Wiktionary
n. 1 (context literature rhetoric English) Referring to something by its polar extremes, as in "we searched high and low". 2 (context literature rhetoric English) Referring to something by a list of its parts.
Wikipedia
In law, a merism is a figure of speech by which a single thing is referred to by a conventional phrase that enumerates several of its parts, or which lists several synonyms for the same thing.
In rhetoric a merism is the combination of two contrasting words, to refer to an entirety. For example, when we mean to say that someone searched thoroughly, everywhere, we often say that someone searched high and low. The title of the tabletop role-playing game Dungeons & Dragons is also a merism. It refers to the game's fantasy theme. Another example of a merism the sword-and-sandal movie genre. It is a loose term for movies taking place in Classical antiquity which were made until 1970.
Merisms also figure in a number of familiar English expressions. The phrase''' lock, stock, and barrel '''originally referred to the parts of a gun, by counting off several of its more conspicuous parts; it has come to refer to the whole of anything that has constituent parts. Basically, {phrase} and everything in between - all encompassing.
In biology, a merism is a repetition of similar parts in the structure of an organism (Bateson 1894). Such features are called meristic characters, and the study of such characters is called meristics.
Merisms are conspicuous features of Biblical poetry. For example, in Genesis 1:1, when God creates "the heavens and the earth" ( KJV), the two parts combine to indicate that God created the whole universe. Similarly, in Psalm 139, the psalmist declares that God knows "my downsitting and mine uprising", indicating that God knows all the psalmist's actions. In addition, Genesis 1:5 uses "evening" and "morning" as a merism for "one day".