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1.
ACI
Anal Cranial Inversion: One's head lodged firmly up into their rectum.
Don't ask for her advice, she's got ACI
by 12kcolg September 18, 2008
 
2.
ACI
Anus Cleaning Incident

An incident during showering when one cleans his/her anus and water penetrates the asshole accidentally.
Mike: Dude, I had the worse ACI today. Almost pooped in the shower.
Rob: You should have used a low flow shower head to clean your ass.
by innoslang September 03, 2014