bbl

Often thought to be modified from bl for barrel (b⸺l), reflecting a way of abbreviating that was used especially in previous centuries (compare do for ditto), wherein a doubled letter indicated plural count, as also with p.=page but pp.=pages and l.=line but ll.=lines; this method was used not only in English but also in scholarly New Latin and other modern languages heavily affected thereby, such as with Spanish EE. UU. According to this idea, addition of the suffix -s, as sometimes seen (e.g., 5 bbls or 5 bbls.), would seem to have come from writers who left the unit abbreviation itself unanalyzed (unparsed), thus adding a venially redundant marker of plural, somewhat like with MPGs for miles per gallon. Some sources claim that the abbreviation bbl is from Standard Oil's 42-gallon blue barrels. But this is contested: one source calls this a "persistent oilfield myth" and notes that "the abbreviation 'bbl' had been in use before the 1859 birth of the petroleum industry". Nonetheless, Standard Oil's blue barrels may have helped the bbl abbreviation stick.

来源：wiktionary