Etymology
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invisible (adj.)
mid-14c., "not perceptible to sight, incapable of being seen," from Old French invisible (13c.), from Latin invisibilis "unseen, not visible," from in- "not" (see in- (1)) + visibilis (see visible). Meaning "kept out of sight" is from 1640s. As a noun, "things invisible," from 1640s. Invisible Man is from H.G. Wells's novel (1897); invisible ink is from 1680s. Related: Invisibly.
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invisibility (n.)
1560s, from Late Latin invisibilitas, from Latin invisibilis "not visible, unseen" (see invisible).
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leaven (v.)
"excite fermentation in," c. 1400, leueyn, from leaven (n.). Figurative sense "work upon by invisible or powerful influence" is from 1540s. Related: Leavened; leavening.
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antichthon (n.)

c. 1600, antichthones (plural), from Latin antichthontes, from Greek antikhthontes "people of the opposite hemisphere," from anti "opposite" (see anti-) + khthōn "land, earth, soil" (from PIE root *dhghem- "earth"). In Pythagorean philosophy, an imagined invisible double of earth.

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inconspicuous (adj.)
1620s, "invisible," from Late Latin inconspicuus "not conspicuous," from in- "not, opposite of, without" (see in- (1)) + Latin conspicuus (see conspicuous). Weakened sense of "not readily seen or noticed" developed by 1828. Related: Inconspicuously; inconspicuousness.
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unsightly (adj.)
1540s, "displeasing to the eye," from un- (1) "not" + sight (n.) + -ly (1). Similar formation in Middle Dutch onsichtlijc "invisible; ugly," Middle High German unsihtlih "invisible." Related: Unsightliness.

Middle English sightlie is attested from mid-15c. but only in the sense "visible;" unsightly is attested in Middle English only as an adverb meaning "invisibly" (late 15c.). Sightly as "pleasing to the eye" is from 1560s. Middle English also had unsighty "difficult or displeasing to look at" (early 15c., from sighty "attractive," late 14c.), also unsightily in the same sense (c. 1400).
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black market (n.)
"unauthorized dealing in restricted or rationed commodities," 1931, from black (adj.), probably suggesting "dark, invisible" or "shady, improper" + market (n.). As an adjective by 1935. It exploded in popularity with the coming of World War II rationing.
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undermine (v.)

c. 1300, undermyne, "render unstable by digging at the foundation," from under + mine (v.1) "dig." The figurative sense "injure by invisible, secret, or dishonorable means" is attested from early 15c. Similar formation in Dutch ondermijnen, Danish underminere, German unterminiren. The Old English verb was underdelfan. Related: Undermined; undermining.

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perspiration (n.)

1610s, "a breathing through," a sense now obsolete, from French perspiration (1560s), noun of action from perspirer "perspire," from Latin perspirare "blow or breathe constantly," from per "through" (from PIE root *per- (1) "forward," hence "through") + spirare "to breathe, blow" (see spirit (n.)). Applied by 1620s to "excretion of invisible moistures through the skin," hence its later use as a euphemism for "sweat" (1725).

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psycho- 

word-forming element meaning "mind, mental; spirit, unconscious," from Greek combining form of psykhē "the soul, mind, spirit; life, one's life, the invisible animating principle or entity which occupies and directs the physical body; understanding, the mind (as the seat of thought), faculty of reason" (see psyche). It also was used to form compounds in Greek, such as psychapates "soul-beguiling" (with apate "deceit").

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