Origins
The word 'explain' is built on one of the most physical metaphors in the language of thought: to make something clear is to flatten it out.βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ Latin 'explanare' compounds 'ex-' (out) with 'planus' (flat, level), producing a verb that literally means 'to spread out flat' β to take something folded, crumpled, or obscure and lay it out so that every part is visible and level.
Latin 'planus' descends from PIE *plehβ-, meaning 'flat' or 'to spread,' one of the most productive roots in the Indo-European family. In the Italic branch, it produced 'planus' (flat, clear, obvious), 'planities' (a flat surface), and 'planum' (a level place). Through French, these gave English 'plain' (flat land; simple, clear), 'plane' (a flat surface; a tool for making things flat), 'plan' (a design laid out flat), 'platform' (a flat form), and 'plateau' (a high flat area).
In the Germanic branch, *plehβ- underwent regular sound changes to produce Proto-Germanic *flataz, the ancestor of English 'flat.' The connection between 'explain' and 'flat' is thus not merely metaphorical but etymological: both derive from the same ancient root meaning 'spread out level.' 'Floor' (from Proto-Germanic *flΕrΕ) is another Germanic descendant, as is possibly 'field' (a flat expanse of land).
Latin Roots
The Greek branch produced 'platys' (flat, broad), which gave English 'plate' (a flat dish), 'plaza' (a flat open space, via Spanish), 'platitude' (a flat remark β something boringly level), and 'platinum' (named for its flat, sheet-like appearance in ore). 'Plato' β the philosopher's nickname β meant 'broad' (referring to either his broad shoulders or his broad forehead).
The metaphorical dimension of 'explain' is worth dwelling on. The conceptual metaphor UNDERSTANDING IS SEEING, and more specifically CLARITY IS FLATNESS, runs deep in Western thought. We speak of 'laying out' an argument, 'unfolding' a theory, 'mapping' a concept, and 'spelling out' the details. All these expressions share the underlying image of taking something complicated and spreading it out on a flat surface where it can be inspected. 'Explain' is the most literal version of this metaphor: to ex-plain is to de-fold, to make plain, to convert a three-dimensional tangle into a two-dimensional map.
The word entered English in the early fifteenth century. The earlier form was 'explaine' or 'explane,' closer to the Latin. The modern spelling 'explain' represents a compromise between the French pronunciation and the Latin root. The noun 'explanation' (from Latin 'explanatio') arrived around the same time.
French Influence
A closely related word is 'display,' from Old French 'despleier' (to unfold), from Latin 'displicare' (to scatter, to unfold). Though 'display' uses a different Latin root ('plicare,' to fold, rather than 'planus,' flat), it shares the same core metaphor: to make visible by unfolding, to reveal by spreading out.
The contemporary use of 'explain' is almost entirely abstract β we explain ideas, not physical objects. But the physical metaphor remains embedded in the word's structure. Every explanation is, at its etymological root, an act of flattening: taking the crumpled, the folded, the obscure, and pressing it smooth until it lies plain before the mind's eye.
The phrase 'self-explanatory' β meaning something so clear it needs no explanation β contains a delightful redundancy at the etymological level: something that flattens itself out, that is already plain. The word family of *plehβ- thus creates a self-referential loop in English: what is plain needs no explaining, and to explain is to make plain.