breakdown (n.)

Old English brecan "to divide solid matter violently into parts or fragments; to injure, violate (a promise, etc.), destroy, curtail; to break into, rush into; to burst forth, spring out; to subdue, tame" (class IV strong verb; past tense bræc, past participle brocen), from Proto-Germanic *brekanan (source also of Old Frisian breka, Dutch breken, Old High German brehhan, German brechen, Gothic brikan), from PIE root *bhreg- "to break."

Closely related to breach (n.), brake (n.1), brick (n.). The old past tense brake is obsolete or archaic; past participle is broken, but shortened form broke is attested from 14c. and was "exceedingly common" [OED] 17c.-18c.

Of bones in Old English. Formerly also of cloth, paper, etc. Meaning "escape by breaking an enclosure" is from late 14c. Intransitive sense "be or become separated into fragments or parts under action of some force" is from late 12c. Meaning "lessen, impair" is from late 15c. Meaning "make a first and partial disclosure" is from early 13c. Meaning "destroy continuity or completeness" in any way is from 1741. Of coins or bills, "to convert to smaller units of currency," by 1882. In reference to the heart from early 13c. (intransitive); to break (someone's) heart is late 14c.

Break bread "share food" (with) is from late 14c. To break ground is from 1670s as "to dig, plow," from 1709 in the figurative sense "begin to execute a plan." To break the ice "overcome the feeling of restraint in a new acquaintanceship" is from c. 1600, in reference to the "coldness" of encounters of strangers. Break wind first attested 1550s. To break (something) out (1890s) probably is an image from dock work, of freeing cargo before unloading it.

The ironic theatrical good luck formula break a leg (by 1948, said to be from at least 1920s) has parallels in German Hals- und Beinbruch "break your neck and leg," and Italian in bocca al lupo. Evidence of a highly superstitious craft (see Macbeth). According to Farmer & Henley, in 17c. the expression was used euphemistically, of a woman, "to have a bastard."

"in a descending direction, from a higher to a lower place, degree, or condition," late Old English shortened form of Old English ofdune "downwards," originally of dune "off from (the) hill," from dune "from the hill," dative of dun "hill" (see down (n.2)). The "hill" word is general in Germanic, but this sense development is peculiar to English. As a preposition, "in a descending direction upon or along,"  from late 14c.

To be down on "express disapproval of" is by 1851. Down home is from 1828 as "in one's home region," as an adjective phrase meaning "unpretentious" by 1931, American English. Down the hatch as a toast is from 1931. Down to the wire is 1901, from horse-racing.

Down Under "Australia and New Zealand" attested from 1886; Down East "Maine" is from 1825; Down South "in the Southern states of the U.S." is attested by 1834. Down the road "in the future" is by 1964, U.S. colloquial. Down-to-earth "everyday, ordinary, realistic" is by 1932.