mark-down n.

"to put a mark on," Old English mearcian (West Saxon), merciga (Anglian) "to trace out boundaries;" in late Old English "make a mark or marks on," from Proto-Germanic *markojan (source also of Old Norse merkja, Old Saxon markon "appoint, observe, remark," Old Frisian merkia, Old High German marchon "to limit, plan out," German merken "to mark, note," Middle Dutch and Dutch merken "to set a mark on"), from the root of mark (n.1).

Influenced by the Scandinavian cognates. Meaning "to have a mark" is from c. 1400; that of "to notice, observe" is late 14c. Figurative sense of "designate as if by placing a mark on," hence "to destine," is from late Old English. Meaning "be a noteworthy feature of" is by 1660s. To mark time (1833) is from military drill, originally "move the feet as if marching but remain in place."

The verbs in Romanic are from the nouns, which are early borrowings from Germanic: Old French merchier "to mark, note, stamp, brand," French marquer "to mark," Spanish marcar, Italian marcare.

"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.