What does "down" mean?

In addition to the idioms beginning with down, also see back down; batten down the hatches; bear down; beat down; be down; belt down; bog down; boil down to; break down; breathe down one's neck; bring down; bring down the house; buckle down; build down; burn down; call down; cast down; caught with one's pants down; chow down; clamp down; close down; come down; come down on; come down to; come down with; cool down; cool off (down); count down; crack down; cut down; deep down; die away (down); dig down; draw down; dressing down; face down; fall down; flag down to; get down to brass tacks; go down (downhill); go down the line; hand down; hands down; hold down; it's all downhill; jump down someone's throat; keep down; knock back (down); knock down with a feather; knuckle down; lay down; lay down the law; lead down the garden path; let down easy; let one's hair down; let someone down; let the side down; lie down (on the job); live down; look down on; lowdown, get the; mark down; mow down; nail down; pin down; pipe down; play down; plunk down; pull down; put down; put down roots; put one's foot down; ram down someone's throat; ring down the curtain; rub down; run down; scale down; sell down the river; send down; set down; settle down; shake down; shoot down; shout down; shut down; simmer down; sit down; slap down; slow down; splash down; stand down; stare down; step down; strike down; suit down to the ground; take down; take down a notch; take lying down; talk down to; tear down; the lowdown on; throw down the gauntlet; thumbs up (down); tie down; tone down; touch down; track down; trade down; turn down; turn upside down; ups and downs; vote down; wash down; water down; wear down; weigh down; when it comes (down) to; when the chips are down; wind down; write down.