A type of lubricant uses to pull wire. Also called soap, pookie and man slime. It can also be referred to sperm, jizz, and most commonly referred to - the moist collection section of a man's taint.
"Jacob, get over here and put some boy butter on the wire while you feed it into the conduit."
"Get some real boy butter by rubbing your balls and taint to feed the wire with some of that workin' man's motion lotion! That will work much better, Jason!"
As with wifebeater, the name comes from the absence of sleeves, which allows the wearer to beat her boy (or perhaps any boy) unencumbered. This usage was first documented at the Goatwhore merchandise table during their 2012 tour.
After seeing the band's set, she wanted to get their boybeater, but they only had them in large.