At the bottom at English => English (Websters 1913) Of Explained:

Bottom \Bot"tom\ (b[o^]t"t[u^]m), n. [OE. botum, botme, AS.
botm; akin to OS. bodom, D. bodem, OHG. podam, G. boden,
Icel. botn, Sw. botten, Dan. bund (for budn), L. fundus (for
fudnus), Gr. pyqmh`n (for fyqmh`n), Skr. budhna (for
bhudhna), and Ir. bonn sole of the foot, W. bon stem, base.
[root]257. Cf. 4th {Found}, {Fund}, n.]
1. The lowest part of anything; the foot; as, the bottom of a
tree or well; the bottom of a hill, a lane, or a page.

Or dive into the bottom of the deep. --Shak.

2. The part of anything which is beneath the contents and
supports them, as the part of a chair on which a person
sits, the circular base or lower head of a cask or tub, or
the plank floor of a ship's hold; the under surface.

Barrels with the bottom knocked out. --Macaulay.

No two chairs were alike; such high backs and low
backs and leather bottoms and worsted bottoms. --W.
Irving.

3. That upon which anything rests or is founded, in a literal
or a figurative sense; foundation; groundwork.

4. The bed of a body of water, as of a river, lake, sea.

5. The fundament; the buttocks.

6. An abyss. [Obs.] --Dryden.

7. Low land formed by alluvial deposits along a river;
low-lying ground; a dale; a valley. ``The bottoms and the
high grounds.'' --Stoddard.

8. (Naut.) The part of a ship which is ordinarily under
water; hence, the vessel itself; a ship.

My ventures are not in one bottom trusted. --Shak.

Not to sell the teas, but to return them to London
in the same bottoms in which they were shipped.
--Bancroft.

{Full bottom}, a hull of such shape as permits carrying a
large amount of merchandise.

9. Power of endurance; as, a horse of a good bottom.

10. Dregs or grounds; lees; sediment. --Johnson.

{At bottom}, {At the bottom}, at the foundation or basis; in
reality. ``He was at the bottom a good man.'' --J. F.
Cooper.

{To be at the bottom of}, to be the cause or originator of;
to be the source of. [Usually in an opprobrious sense.]
--J. H. Newman.

He was at the bottom of many excellent counsels.
--Addison.

{To go to the bottom}, to sink; esp. to be wrecked.

{To touch bottom}, to reach the lowest point; to find
something on which to rest.

Related words:

at table  at that  at that place  at the best  at the best of times  at the coal face  at the corner  At the countretaille  at the double  At the downlying  at the drop of a hat  At the elbow  at the eleventh hour  at the end of one's rope  at the end of one's tether  at the end of one’s tether  at the end of the day  at the end of the road  at the end of your rope  at the expense of  at the feet of  At the first  At the first blush  at the foot of  At the fore  At the hand of  at the hands of  at the helm  at the high port  At the last  At the last gasp  at the last minute  at the latest  at the least  At the long run  at the mercy of  at the moment  at the most  at the ready  at the receiving end  at the same time  at the start  at the top of one's lungs  at the top of one's voice  at the wheel  at the worst  at this moment  at this moment in time  at this point in time  At times  át tudod ugrani?