Origins
The English adjective 'true' has an etymology that illuminates the deep history of how human beings have thought about truth itself. It descends from Old English 'trēowe,' meaning 'faithful, trustworthy, honest, steadfast,' from Proto-Germanic *triwwiz, from the PIE root *drewh₂-, meaning 'firm, solid, steadfast.' This same root is the source of English 'tree' — the archetypal firm, upright, rooted thing. To be true, in the original Indo-European conception, was to be as solid and reliable as a tree.
This connection between truth and firmness, between honesty and rootedness, pervades the entire word family. 'Trust' (from Old Norse 'traust,' itself from *drewh₂-) is the confidence one places in something firm. 'Truce' (from Old English 'trēow,' a pledge) was originally a covenant of faithfulness. 'Troth' (as in 'plight one's troth') is the pledge of loyalty itself. 'Betroth' means to pledge someone in a covenant. All of these words — true, trust, truce, troth, tree — are siblings from the same PIE root, bound together by the concept of steadfastness.
The semantic history of 'true' reveals a profound shift in the Western concept of truth. In Old English and through much of the Middle English period, 'trēowe' primarily described people, not propositions. A true knight was a loyal knight. A true friend was a faithful friend. A true love was a constant love. Truth was a moral quality — steadfastness, reliability, keeping one's word. The modern primary sense of 'true' — 'in accordance with fact, corresponding to reality' — developed gradually through the later medieval period, influenced partly by the philosophical traditions of scholasticism, which foregrounded truth as a property of statements rather than of persons.
Development
This older sense of personal fidelity is preserved in many surviving expressions: 'true love' (faithful love, not just 'real' love), 'true to one's word,' 'tried and true,' 'true-hearted,' and 'true blue' (originally describing a dye that did not fade, hence faithful and constant). The Wedding vow to be 'true' to one's spouse preserves the original meaning with crystalline clarity — it means loyal, not factually accurate.
The Proto-Germanic cognates consistently preserve the 'faithful, loyal' sense rather than the 'factually correct' sense. German 'treu' means 'faithful, loyal' (as in 'ein treuer Freund,' a loyal friend) and does not mean 'factually true' — for that, German uses 'wahr.' Dutch 'trouw' similarly means 'faithful' or 'loyal.' Old Norse 'tryggr' meant 'trusty, faithful, safe.' Gothic 'triggws' meant 'faithful' or 'covenanted.' In none of these languages did the cognate develop the propositional-truth meaning that English 'true' has — this appears to be a uniquely English semantic innovation.
The abstract noun 'truth' is formed from 'true' with the suffix '-th' (Proto-Germanic *-iþō), the same suffix in 'youth,' 'health,' 'growth,' and 'length.' Old English 'trēowþ' (truth, faithfulness, pledge) is the direct ancestor, surviving into Middle English as 'trouthe' — the word Chaucer's Knight had on his lips: 'Trouthe and honour, fredom and curteisie.' For Chaucer, 'trouthe' still carried its full Old English weight of faithfulness and integrity.
Middle English
The verb 'trow' (to believe, to trust), now archaic, is another sibling from the same root. 'I trow' meant 'I believe' or 'I trust,' and it appears frequently in Middle English and Early Modern English texts. Its disappearance from standard English has left 'true' as the last common survivor of its word family in everyday speech, though 'trust' (borrowed from Norse) carries on the theme.
The technical use of 'true' in carpentry and engineering — a surface is 'true' if it is perfectly flat, a wheel is 'true' if it runs without wobble — preserves something close to the PIE original. Here 'true' means not 'factually correct' but 'firm, straight, aligned' — the same physical steadfastness that *drewh₂- described six thousand years ago. The process of straightening a warped wheel is still called 'truing,' a word that connects the modern bicycle mechanic to the ancient Indo-Europeans' intuition that truth and physical firmness are the same thing.