A state of (un)dress where the
bottom (see also
posterior,
butt,
rear end,
ass) is not covered. Makes sense, huh?
This phrase is most often used in connection with the word spanking, as in "bare bottom spanking" or "being spanked bare bottom". A spanking is something a kid gets accustomed to after receiving enough of them, especially if it's "just" a hand spanking on the seat of the pants, as I used to get at school(*1) lo these many years ago. But a bare bottom spanking, now that was good for an extra dose of terror, two simple words to turn a nervous stomach into a hard knot and make one's hand instinctively reach out to the soon-to-be target area. The words "bare bottom" would turn an everyday spanking into a "holiday" spanking; more dread, more humiliation, more pain.
If, on the other hand, you happen to be into BDSM or just a little more excitement in your foreplay, then a bare bottom spanking will still provide similar feelings to the recipient, but usually delightfully intermixed with such sensations as arousal or just plain horniness. The pain becomes a turn-on, the humiliation caters to exhibitionism. All the while, to the spanker, this act offers an opportunity to deal extensively with an erogenous zone of a loved one, and a power trip without any qualms about possible child abuse.
In fact, in my opinion, spankings (especially bare bottom spankings) are much better suited for adults than children.
(*1) Before you ask, that school was a military school and it no longer exists.