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a shortened form of the word 'Borderline' which is used to indicate a joke or statement being sick and/or wrong, ie on the borderline of going too far in terms of general decency.
'i bet that bloke likes to rape young children in his spare time!'
'oh no! thats BL!'
by Simon Stafford May 28, 2005
In Japan, BL is the "modern" term for Shounenai, deffinition meaning "Boys love."
No matter how you spell it, english or kanji, you will get NOTHING on shounenai on a Japanese search engine, save dead sites refurring to retro mangas from the 70s & 80s.
by Allaiyah January 14, 2004
Beautiful Lineswoman

From Seinfeld, "The Lip Reader"

Jerry: She is stunning.
George: Who, the tennis player?
Jerry: No. The lineswoman.
George: Yeah, she is one B.L.
Jerry: What's a B.L.?
George: A beautiful lineswoman.
by lighthouse_keeper April 13, 2009
Brother n Law ...
"What up BL... ?"
by Kila_T June 02, 2014
abbreviation of beltlooping, the act of concealing an erection behind your belt or waistband
You should BL in case you get asked to stand in front of the class
by tory botra May 11, 2013
Abbreviation for Beautiful Lineswoman - a lineswoman at a tennis match that is attractive.

*Taken from the Seinfeld episode "The Lip Reader"*
Jerry: I can't get over how beautiful that lineswoman is.
George: She's a BL.
Jerry: A BL?
George: Yeah, a BL, a beautiful lineswoman.
by rjgoldsborough June 06, 2010
Barely Legal. A person who has just recently became of age.
She's good to go. She just went from JB to BL.
by Lauren Gio December 28, 2005
ball lightning, the most deadly force in all three dimensions
how the fuck did you make that hypercube? BL bro
by taz gam March 20, 2004