Wikipedia reading: Metaphysical conceit.
In literature, a conceit is an extended metaphor with a complex logic that governs a poetic passage or entire poem. By juxtaposing, usurping and manipulating images and ideas in surprising ways, a conceit invites the reader into a more sophisticated understanding of an object of comparison. Extended conceits in English are part of the poetic idiom of Mannerism, during the later sixteenth and early seventeenth century.
In English literature the term is generally associated with the 17th century metaphysical poets, an extension of contemporary usage. In the metaphysical conceit, metaphors have a much more purely conceptual, and thus tenuous, relationship between the things being compared. Helen Gardner observed that “a conceit is a comparison whose ingenuity is more striking than its justness” and that “a comparison becomes a conceit when we are made to concede likeness while being strongly conscious of unlikeness.” An example of the latter would be George Herbert's “Praise (3),” in which the generosity of God is compared to a bottle which (“As we have boxes for the poor”) will take in an infinite amount of the speaker's tears.
An often-cited example of the metaphysical conceit is the metaphor from John Donne's “The Flea”, in which a flea that bites both the speaker and his lover becomes a conceit arguing that his lover has no reason to deny him sexually, although they are not married:Oh stay! three lives in one flea spare Where we almost, yea more than married are. This flea is you and I, and this Our marriage-bed and marriage-temple is.
When Sir Philip Sidney begins a sonnet with the conventional idiomatic expression “My true-love hath my heart and I have his”, but then takes the metaphor literally and teases out a number of literal possibilities and extravagantly playful conceptions in the exchange of hearts, the result is a fully formed conceit.
The Petrarchan conceit, used in love poetry, exploits a particular set of images for comparisons with the despairing lover and his unpitying but idolized mistress. For instance, the lover is a ship on a stormy sea, and his mistress “a cloud of dark disdain”; or else the lady is a sun whose beauty and virtue shine on her lover from a distance.
The paradoxical pain and pleasure of lovesickness is often described using oxymoron, for instance uniting peace and war, burning and freezing, and so forth. But images which were novel in the sonnets of Petrarch became clichés in the poetry of later imitators. Romeo uses hackneyed Petrarchan conceits when describing his love for Rosaline as “bright smoke, cold fire, sick health”.
History of the term
In the Renaissance, the term (which is related to the word concept) indicated any particularly fanciful expression of wit, and was later used pejoratively of outlandish poetic metaphors.
Recent literary critics have used the term to mean simply the style of extended and heightened metaphor common in the Renaissance and particularly in the 17th century, without any particular indication of value. Within this critical sense, the Princeton Encyclopedia makes a distinction between two kinds of conceits: the Metaphysical conceit, described above, and the Petrarchan conceit. In the latter, human experiences are described in terms of an outsized metaphor (a kind of metaphorical hyperbole), like the stock comparison of eyes to the sun, which Shakespeare makes light of in his sonnet 130: “My mistress' eyes are nothing like the sun.”
And here's Shakespeare's sonnet 130, incredibly interesting:
My mistress' eyes are nothing like the sun; Coral is far more red than her lips' red ; If snow be white, why then her breasts are dun; If hairs be wires, black wires grow on her head. I have seen roses damask, red and white, But no such roses see I in her cheeks; And in some perfumes is there more delight Than in the breath that from my mistress reeks. I love to hear her speak, yet well I know That music hath a far more pleasing sound; I grant I never saw a goddess go; My mistress, when she walks, treads on the ground: And yet, by heaven, I think my love as rare As any she belied with false compare. –William Shakespeare
This poem, standalone by itself, but is a good illustration of literature's concept of “conceit” by mockery.