All my life I've heard "the bottom line" meaning that which is most important or profitable, but what is its origin?
closed as off-topic by choster, user66974, Edwin Ashworth, Davo, Dan Bron Sep 29 '17 at 11:49
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As has been described in the existing answers, the logic behind the phrase "bottom line" derives from the "sum" or "total" line at the bottom of a financial document. The OED attests this sense of the phrase in 1831, so it is a relatively old use.
To be clear, the definition attested in 1831 is defined as:
2.b. Originally: the last line of an account, bill, etc., showing the final total or balance; a final total or balance shown in this way. Later: spec. the last line of a profit and loss account, showing the final profit (or loss); (also more generally) a net profit.
Then there is another definition that is less technical, which is attested earlier (1830), but the OED asserts in a note that it is likely a figurative derivation of the sense described above.
2.a. The fundamental and most important or determining factor; the essence, the point, the crux of the argument; the final analysis, conclusion, or outcome, esp. after a debate.
On the other hand, Etymonline provides a rather odd suggestion for dating:
bottom line (n.)
figurative sense is attested from 1967, from profit-and-loss accounting, where the final figure after both are calculated is the bottom line on the page. Also (especially as an adjective) bottom-line, bottomline.
I suspect that this is a case of Etymonline being in error. They seem to be using an attestation date provided by OED for unique compound variants of the word.
C1. General attrib. with sense ‘that is or represents the bottom line’ (in various senses), esp. ‘of, relating to, or designating the final or net profit, outcome, or level’; as bottom line growth, bottom line result, etc.
It seems possible that Etymonline took this attestation and erroneously applied it to the phrase "bottom line" meaning either of the definitions described at the top of this answer, although that explanation doesn't account for the 1967 dating in Etymonline and the 1968 dating in the OED. Regardless, OED attests both senses of the phrase more than a century earlier. The only later senses that OED offers are one that is unique to music, and another meaning
A minimum acceptable level or standard.
... neither of which applies to this question, nor does either seem to be what Etymonline was referring to when it wrote:
figurative sense is attested from 1967, from profit-and-loss accounting, where the final figure after both are calculated is the bottom line on the page.
Note the similarity of the definition provided to the OED definition 2.b., and the wide discrepancy between dates cited.
Of course, the other possibility is that Etymonline is referring to a sense or variant of the phrase that is unique from any of the senses attested by the OED. As suggested in the comments, perhaps it refers only to "the last line of a profit and loss account," which the OED definition refers to as a "later" variant of the sense attested in 1831. Because the definition in Etymonline is so brief and doesn't provide a citation for its 1967 attestation, I find it difficult to pin down where they came up with that date, and I would argue that the dates and citations provided in OED give a more accurate picture of the origin of the phrase "bottom line" in the sense referred to in the question.
It originates from accounting, from a financial context. The bottom line is where you find the result of a (possibly long) calculation.
123 + 345 —————— = [the bottom line]
So in a figurative sense, the bottom line is the result, the conclusion or the essence: That what matters in the end.
An early instance of "the bottom line" used in a figurative (and very modern-sounding) way appears in "New Movement to 'Supersede the Liberty Party'," in the [Salem, Ohio] Anti-Slavery Bugle (March 26, 1847):
Some, of course, who were very zealous for the destruction of Southern Slavery, were not prepared for this direct method [ending "the monopoly of the soil"], which would also upset something very nearly allied to it, by which they in the North had been profiting; and hence the new movement to "supersede the Liberty party." Mr. Goodell says, "It has been plausible, all along, to talk of voting 'irrespective of party,' but the bottom line of the policy always is, to vote with the tariff party, on the plea that they are the most favorable of the two," and he concludes as follows: ...
So we have a 170-year-old example in which "the bottom line" means "the ultimate point, goal, substance, or result [of a policy]."
Most financial documents/statements have the most important number at or near the bottom. Income statements have the net income on the bottom line. Balance sheets have total assets and liabilities on the bottom line.