1.) To quickly bathe or shower, following the example of a bird, hence the name. Does not involve soap or shampoo, simply water. Typically done after strenuous activity causes one to sweat, and thusly, stink.
2.) To wash one's penis in a sink, in the situation that one might 'get lucky' that night and is worried that their penis may have a strange or unpleasant odor. So called because of the reference to bird meaning genitalia.
1.) OH shit I just shoveled 18 inches of snow and forgot deodorant. I'd best go and have a birdbath.
2.)OH shit, this chicksaid YES to go back to my place. I'd better go to the restroom and have a birdbath.
Achieved by taking your lady friend from behind, either on her knees or stomach. One pulls out prior to busting, aiming their shaft in the direction of the small of her back and proceeds to unload. The small of her back will cup the load allowing a pooling effect which is known as a birdbath.
“threeguyslater, her birdbath overflowed and ran down the side of her leg.”
- The act of pulling out, mid - coitus, and ejaculating on you partner's back; specifically, the dimples of the back, therefore creating a "bird bath" effect.
When you're fucking a girl from behind and you pull out and cum in that inverted spot in the small of her back, filling it with cum, and then proceed to shake like a bird drying themselves off as you ejaculate.
She has the sexiest back I've ever seen. I'm going to give her a birdbath later.