Etymology
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impracticable (adj.)
"incapable of being done, not to be done by available means," 1670s, from assimilated form of in- (1) "not, opposite of" + practicable. Earlier in a sense of "impassable" (1650s). Related: Impracticably; impracticability.
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impractical (adj.)
1823, from assimilated form of in- (1) "not, opposite of" + practical (adj.). Impracticable in the same sense dates from 1670s; unpractical is rare. Related: Impractically.
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ineffectible (adj.)
"that cannot be carried out, impracticable," 1803, from in- (1) "not, opposite of" + effectible (see effect (v.)).
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no-go (adj.)

"where it is forbidden to go," 1971, from no + go (v.). Earlier it was a noun phrase for an impracticable situation (1870) and a type of horse race (by 1860).

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non-starter (n.)

also nonstarter, "one who does not start a race, contest, etc.," hence "ineffectual person or impracticable idea," 1909, from non-  + starter.

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aporetic (adj.)
"inclined to doubt," c. 1600, from French aporetique, from Greek aporetikos, from aporeein "to be at a loss, be without means or resources," from aporos "impassable, impracticable, very difficult; hard to deal with; at a loss," from a- "not, without" (see a- (3)) + poros "passage" (see pore (n.)).
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aporia (n.)

1580s, in rhetoric, "professed doubt as to where to begin," from Latin, from Greek aporia "difficulty, perplexity, want of means, poverty," abstract noun from aporos "impassable, impracticable, very difficult; hard to deal with; at a loss," from a- "not, without" (see a- (3)) + poros "passage" (from PIE root *per- (2) "to lead, pass over"). Meaning "equality of reasons for or against" is by 1893.

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foul (v.)
Old English fulian "to become foul, rot, decay," from ful (see foul (adj.)). Transitive meaning "make foul, pollute" is from c. 1200. Meaning "become entangled" (chiefly nautical) is from 1832, probably from foul (adj.) in the sense "obstructed by anything fixed or attached" (late 15c.). "A term generally used in contrast to clear, and implies entangled, embarrassed or contrary to: e.g. to foul the helm, to find steerage impracticable owing to the rudder becoming entangled with rope or other gear" [Sir Geoffrey Callender, "Sea Passages," 1943]. Related: Fouled; fouling. Hence also foul anchor (1769), one with the slack of the cable twisted round the stock or a fluke; noted by 1832 as naval insignia.
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viaduct (n.)

1816, from Latin via "road" (see via) + -duct as in aqueduct. French viaduc is a 19c. English loan-word.

An extensive bridge consisting, strictly of a series of arches of masonry, erected for the purpose of conducting a road or a railway a valley or a district of low level, or over existing channels of communication, where an embankment would be impracticable or inexpedient; more widely, any elevated roadway which artificial constructions of timber, iron, bricks, or stonework are established. [Century Dictionary]

But the word apparently was coined by English landscape gardener Humphry Repton (1752-1818) for an architectural feature, "a form of bridge adapted to the purposes of passing over, which may unite strength with grace, or use with beauty ...."

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