Britney Spears posted a brand spankin' new dancing video ... and, we mean that quite literally, 'cause she's giving her bottom some crisp smacks in the clip.

The singer-songwriter shared the video to Instagram Monday afternoon ... swiveling her hips around in a circle while wearing what looks like just gray lingerie. Instagram / @britneyspears

A flower emoji pops up on Brit's chest periodically ... 'cause the chest fabric's so see-through it seems her nipples are fully visible every so often.

While turned away from the camera, Britney delivers a few smacks to her own backside ... we had to mute the clip because of music she's playing -- but, trust us, she ain't holding back. Instagram / @britneyspears

The clip ends with Britney grabbing, then letting go of her breasts several t

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