Present participle of 'bull': to push forcefully, to advance aggressively despite obstacles, or (of cattle) to engage in mating behavior.
From 'bull' as a verb, which comes from the noun. Using animal behavior as a verb allows us to describe human action metaphorically—'bulling through' means acting with bull-like determination.
In finance, 'bulling' the market became so common that 'bull market' is now standard terminology—traders basically borrowed the image of an aggressive charging bull to describe an optimistic, rising market!
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