Origins
The English adjective 'straight' has a beautifully transparent etymology once you see it: it is the past participle of 'stretch.' Something straight is something that has been stretched — pulled taut until it forms a line without curves. The word entered its modern adjectival role through Middle English 'streight' or 'streiȝt,' the past participle of 'strecchen' (to stretch), from Old English 'streccan' (to stretch, to extend, to spread out), from Proto-Germanic *strakjaną (to stretch, to make straight), ultimately from PIE *sterg- meaning 'stiff,' 'rigid,' or 'taut.'
The relationship between 'straight' and 'stretch' is obscured in modern English by centuries of divergent spelling and pronunciation, but it was transparent to Middle English speakers. 'Streight' was simply what you got when you stretched something — the participial form used as an adjective, exactly like 'broken,' 'frozen,' or 'spoken.' The vowel change from 'stretch' (short e) to 'straight' (diphthong ei > modern ay) reflects the regular phonological development of the Old English past participle, where the vowel was often different from the present stem.
The Proto-Germanic cognates preserve the verbal sense. German 'strecken' (to stretch, to extend), Dutch 'strekken' (to stretch), and Swedish 'sträcka' (to stretch) all descend from the same root. German also has 'stracks' (straight, directly), an adverb from the same family. The adjective 'straight' as such is a peculiarly English development — other Germanic languages use different words for the adjectival concept (German 'gerade,' Dutch 'recht').
French Influence
A persistent source of confusion in English is the relationship — or rather, non-relationship — between 'straight' and 'strait.' Despite identical pronunciation and historically overlapping spellings, the two words have entirely different origins. 'Strait' (narrow, constricted, as in the Strait of Gibraltar or 'dire straits') comes from Old French 'estreit,' from Latin 'strictus' (drawn tight), the past participle of 'stringere' (to bind, to draw tight). Both 'straight' and 'strait' involve the concept of tension and tautness, but they arrive at it through separate etymological pathways — one Germanic, one Latin.
The confusion between the two has produced the common misspelling 'straight-laced' for what was originally 'strait-laced' (laced tightly, hence rigidly proper in manners). However, 'straight-laced' has become so common that many dictionaries now accept it as an alternative spelling. The biblical 'strait and narrow' (from Matthew 7:14) properly refers to a narrow, constricted path, not a direct one, though conflation with 'straight' has given the phrase an additional sense of moral directness.
The figurative extensions of 'straight' are extensive and culturally significant. 'Straight' meaning 'honest' or 'direct' (straight talk, straight answer) dates from the sixteenth century, drawing on the association between a straight line and a direct path — no detours, no deception. 'Straightforward' (direct, uncomplicated) compounds this sense. 'To set straight' or 'to put straight' means to correct someone's misunderstanding, returning them to the direct, true path.
Figurative Development
'Straight' as a term for conventional or heterosexual identity emerged in mid-twentieth-century American slang, contrasting with 'bent,' 'crooked,' or 'queer.' The usage draws on the long-standing metaphorical association between straightness and conformity to norms.
In poker, a 'straight' (five cards in sequential order) dates from the 1860s, when the game was being formalized. The name reflects the idea of cards arranged in a direct, unbroken sequence. In boxing, a 'straight right' is a punch delivered in a direct line. In billiards, 'straight pool' involves potting balls in sequence.
The phrase 'the straight and narrow' (a virtuous, disciplined way of life) conflates the biblical 'strait' with the adjective 'straight,' producing a phrase that works on both levels: the path is narrow (strait) and direct (straight). This double meaning has ensured the phrase's survival even as the word 'strait' in its adjectival sense has otherwise become rare.
Middle English
'Straight' as an adverb (go straight, stand straight) has been in use since Middle English and is probably older than the adjectival use. The idea of moving 'straight' — in a direct line, without deviation — is the word's most physical and most ancient application, connecting it back to its origin in the act of stretching.