The usual definition: During anal sex the man punches the woman to tighten up her butt.
The untold rendition of 'donkey punch': A guy is having sex with a girl doggie style (vaginally). Without a single warning, he pulls out and puts it in her butt. When she turns her head around to say 'what the fuck?', he socks her in the back of the head and she shits on his dick.
Have you heard the new way to break up with a girl? Ya, it's called 'donkey punch
'. Cuz then you can tell everyone she shat on your cack.