CBAM (pronounced 'see-BAM') is an acronym for the phrase 'cock-borne ass-matter'. CBAM can result when a couple engages in anal sex, and the recipient's ass is not completely 'prepared' for the invasion of the perpetrator's cock.
Thus, upon withdrawal from the recipient's ass, small amounts of fecal matter may cling to the perpetrator's dick like a rotweiler hangs onto a robber's femur. CBAM, while not entirely visually or aesthetically pleasing, is a necessary evil if one is to perform a dirty sanchez, a dirty rodriguez, a dirty ash wednesday, or the dirty menorah.
Indeed, were it not for CBAM it would be impossible to complete any of the above acts as well as other 'dirty' acts that have recently become so popular in the arena of sexual shenanigans.
"Dude...last night I picked up this waitress and gave her a dirty sanchez and...even after I had painted the Frito-Bandito 'stache on her face...my dick still had CBAM all over it. I think she had nachos for lunch..."
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