A state of (un)dress where the bottom
(see also posterior
, rear end
) 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.