The compound adjective 'well-understood' is a relatively modern formation, built upon the much older verb 'understand'. The verb 'understand' itself has a fascinating history, originating from Old English 'understandan', which literally meant 'to stand among or in the midst of'. This concrete spatial sense evolved metaphorically to 'comprehend' by the 12th century, suggesting the mind 'standing in the midst' of an idea to grasp it fully. The intensifying adverb 'well', meaning 'thoroughly', was prefixed to the past participle 'understood' to create an adjective describing a concept that is not merely known, but known clearly, completely, and without ambiguity. Thus, the word's logic moves from a physical position ('standing under/among') to a state of mental comprehension, and finally to a quality of being thoroughly comprehended.

Structure: well (thoroughly) + under + stood (grasped) - well: Old English origin, from the adverb 'wel' meaning 'in a good way, thoroughly'. In this compound, it functions as an intensifier. - under: Old English origin, from the preposition 'under' meaning 'beneath, among'. In this compound, it is part of the verb 'understand', carrying the sense of being 'among' the facts or details. - stood: Old English origin, the past participle of 'stand' (standan). In the verb 'understand', it evolved to mean 'to grasp, to comprehend'. Here, it functions as the core verbal element meaning 'grasped'.