Definition of "sour"

All sour things, as vinegar, provoke appetite.

1631, Francis [Bacon], “(please specify |century=I to X)”, in Sylua Syluarum: Or A Naturall Historie. In Ten Centuries. […], 3rd edition, London: […] William Rawley; [p]rinted by J[ohn] H[aviland] for William Lee […]

A few types of molecules get sensed by receptors on the tongue. Protons coming off of acids ping receptors for "sour." Sugars get received as "sweet." Bitter, salty, and the proteinaceous flavor umami all set off their own neural cascades.

2018 May 16, Adam Rogers, “The Fundamental Nihilism of Yanny vs. Laurel”, in Wired

He was a scholar […] / Lofty and sour to them that loved him not, / But to those men that sought him sweet as summer.

c. 1591–1592 (date written), William Shakespeare, “The Third Part of Henry the Sixt, […]”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act III, scene i]

Let me embrace thee, sour adversity

1613 (date written), William Shakespeare, [John Fletcher], “The Famous History of the Life of King Henry the Eight”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act IV, scene ii]

The result may not quite give the Wearsiders a sweet ending to what has been a sour week, following allegations of sexual assault and drug possession against defender Titus Bramble, but it does at least demonstrate that their spirit remains strong in the face of adversity.

2011 October 1, Phil Dawkes, “Sunderland 2 - 2 West Brom”, in BBC Sport‎

Unlike what the name implies, there is nothing inherently wrong with a sour note: It is perfectly well-tuned note that would sound normal in another context (and which presumably would not sound sour to someone unfamiliar with tonal music).

2010, Aniruddh D. Patel, Music, Language, and the Brain, page 201

For many Years of Sorrow can dispense; A Dram of Sweet is worth a Pound of Sour

1590, Edmund Spenser, “Book I, Canto III”, in The Faerie Queene. […], London: […] [John Wolfe] for William Ponsonbie

So the sun's heat, with different powers, / Ripens the grape, the liquor sours.

1720, Jonathan Swift, To Stella, on transcribing my Poems

To sour your happiness I must report, / The queen is dead.

1611 April (first recorded performance), William Shakespeare, “The Tragedie of Cymbeline”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act V, scene v]

He was prudent and industrious, and so good a husbandman, that he might have led a very easy and comfortable life, had not an arrant vixen of a wife soured his domestic quiet.

1749, Henry Fielding, The History of Tom Jones, a Foundling, volumes (please specify |volume=I to VI), London: A[ndrew] Millar, […]

stagnant water , which tends to sour the soil

1832, Joseph Harrison, Sir Joseph Paxton, The Horticultural Register, page 396