lie-down (n.)

"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.

"rest horizontally, be in a recumbent position," Middle English lien, from Old English licgan (class V strong verb; past tense læg, past participle legen) "be situated, have a specific position; remain; be at rest, lie down," from Proto-Germanic *legjan (source also of Old Norse liggja, Old Saxon liggian, Old Frisian lidzia, Middle Dutch ligghen, Dutch liggen, Old High German ligen, German liegen, Gothic ligan "to lie"), from PIE root *legh- "to lie down, lay."

Especially "to lie in bed," hence often with sexual implications, as in lie with "have sexual intercourse" (c. 1300), and compare Old English licgan mid "cohabit with." To lie in "be brought to childbed" is from mid-15c. To lie to at sea is to come to a standstill. To take (something) lying down "receive passively, receive with abject submission" is from 1854.