/ɪkˈspleɪn/
[verb] To make something clear and easy to understand by giving details, reasons, or examples. If you explain something, you help another person see how or why it works.
To explain something once literally meant to flatten it out — as if ideas were wrinkled cloth that needed to be smoothed.
The word 'explain' arrives in English in the 15th century, but its roots stretch back to classical Latin, where the verb 'explanare' meant to make flat or to spread out on a level surface. This was built from two parts: the prefix 'ex-', meaning out or thoroughly, and 'planus', meaning flat or level. In its earliest Latin use, 'explanare' was almost entirely physical — you would 'explanare' a piece of cloth by pressing it smooth, or 'explanare' a path by leveling the ground. The mind's journey from the physical to the abstract is one of language's most beautiful and recurring tricks.
The metaphor underneath 'explain' is therefore surprisingly tactile. When the Romans began using 'explanare' to describe the act of making an idea clear, they were imagining a thought as something crumpled, folded, or uneven — something that needed to be pressed out and laid flat before it could be properly seen. To explain was to iron out the creases of confusion. This same root, 'planus', gave Latin a rich family of words connected to flatness and openness, which rippled outward through the Romance languages and eventually into English.
From Latin 'planus', Old French inherited 'plain', meaning flat or clear, which passed into Middle English as 'plain' — still very much alive today when we talk about 'plain speech' or a 'plain view'. The same Latin root also produced 'planar' (relating to a flat surface), 'plane' (a flat geometric surface, or a carpenter's tool for smoothing wood), and even the word 'piano' — which reached English via Italian 'pianoforte', literally meaning 'soft-loud', but whose ancestor 'piano' in Italian also carried the sense of flat and level. Every time someone sits down at a piano, they are, in the most distant sense, touching the same root that gives us 'explain'.
The Latin 'explanare' passed into Old French as 'explaner' and then 'esplaner', and it is through this French channel that the word entered Middle English. By the time English writers were using 'explain' in the 1400s and 1500s, the physical sense of flattening had almost entirely disappeared, replaced by the purely intellectual meaning of making something understandable. This shift reflects a common pattern in how abstract vocabulary develops — concrete actions (pressing, smoothing, spreading) become metaphors for mental work (clarifying, simplifying, demonstrating).
Interestingly, the word 'plain' itself, when used to mean clear or simple, carries exactly this same metaphorical logic. To speak plainly is to speak on a level surface, without the bumps and folds of obscurity. 'Explain' simply takes that idea further by adding the intensifying prefix 'ex-', suggesting a more thorough or deliberate act of flattening. The difference between saying something plainly and explaining it is, at the root level, merely a question of how much smoothing the idea requires. It is a remarkably honest metaphor — one that quietly admits that understanding is something you have to work toward, pressing out the wrinkles one by one.