From WordNet (r) 3.0 (2006) :
bbl
n 1: any of various units of capacity; "a barrel of beer is 31
gallons and a barrel of oil is 42 gallons" [syn: barrel,
bbl]
From V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016) :
BBL
Be Back Later (slang, Usenet, IRC)
From The Free On-line Dictionary of Computing (30 December 2018) :
BBL
(I will) be back later.