'BS' as an abbreviation of the slang word 'bullshit' comes from early 20th-century U.S. English, where 'bullshit' was used to mean 'nonsense' or 'rubbish'. Separately, 'B.S.' as an academic abbreviation stands for 'Bachelor of Science' (from the degree name).

'bullshit' developed as a compound of 'bull' (used figuratively to mean nonsense) + 'shit' (expletive/excrement). The initial vulgar compound became common slang; the two-letter abbreviation 'BS' (often written 'B.S.') arose as a euphemistic or shorthand form in writing and speech. The academic 'B.S.' derives independently from naming conventions for university degrees ('Bachelor of Science').

Originally a literal reference to excrement combined with an intensifier, 'bullshit' quickly took on the figurative meaning 'nonsense' and has since been abbreviated to 'BS' in informal usage; the abbreviation 'B.S.' for the degree retained a formal, non-vulgar meaning.