bull (n.1)

"male of a bovine animal," c. 1200, bule, from Old Norse boli "bull, male of the domestic bovine," perhaps also from an Old English *bula, both from Proto-Germanic *bullon- (source also of Middle Dutch bulle, Dutch bul, German Bulle), perhaps from a Germanic verbal stem meaning "to roar," which survives in some German dialects and perhaps in the first element of boulder (q.v.). The other possibility [Watkins] is that the Germanic word is from PIE root *bhel- (2) "to blow, swell."

An uncastrated male, reared for breeding, as opposed to a bullock or steer. Extended after 1610s to males of other large animals (elephant, alligator, whale, etc.). Stock market sense "one who seeks to cause a rise in the price of a stock" is from 1714 (compare bear (n.)). Meaning "policeman" attested by 1859. Bull-necked is from 1640s. Figurative phrase totake the bull by the horns "boldly face or grapple with some danger or difficulty" first recorded 1711 (Swift). To be a bull in a china shop, figurative of careless and inappropriately destructive use of force, attested from 1812 and was the title of a popular humorous song in 1820s England.

bull (n.2)

"papal edict, highest authoritative document issued by or in the name of a pope," c. 1300, from Medieval Latin bulla "sealed document" (source of Old French bulle, Italian bulla), originally the word for the seal itself, from Latin bulla "round swelling, knob," said ultimately to be from Gaulish, from PIE *beu-, a root supposed to have formed a large group of words meaning "much, great, many," also words associated with swelling, bumps, and blisters (source also of Lithuanian bulė "buttocks," Middle Dutch puyl "bag," also possibly Latin bucca "cheek").

bull (v.)

"push through roughly," 1884, from bull (n.1). Related: Bulled; bulling.

bull (n.3)

"insincere, trifling, or deceptive talk," 1914. Popularly associated with roughly contemporary bullshit (n.) in the same sense, and in modern use often felt as a shortened form of it. There seems to have been an identical Middle English word meaning "false talk, fraud," apparently from Old French bole "deception, trick, scheming, intrigue," and perhaps related to modern Icelandic bull "nonsense."

Sais christ to ypocrites ... yee ar ... al ful wit wickednes, tresun, and bull. ["Cursor Mundi," Northumbrian, early 14c.]

There also was an early Modern English verb bull meaning "to mock, cheat," which dates from 1530s. Bull session is attested from 1920.

Also of uncertain connection with the bull that means "a gross inconsistency in language, a ludicrous blunder involving a contradiction in terms" (1630s), said by the English to be characteristic of the Irish, and thus often called an Irish bull. Sydney Smith defined it as "an apparent congruity, and real incongruity of ideas, suddenly discovered." Three examples attributed to Sir Boyle Roche: "Why should we do anything for posterity, for what, in the name of goodness, has posterity done for us?" ... "It would surely be better, Mr. Speaker, to give up not only a part, but, if necessary, even the whole of our Constitution, to preserve the remainder." ... "The best way to avoid danger is to meet it plump."

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Definitions of bull from WordNet
1
bull (n.)
uncastrated adult male of domestic cattle;
bull (n.)
a large and strong and heavyset man;
he was a bull of a man
Synonyms: bruiser / strapper / Samson
bull (n.)
obscene words for unacceptable behavior;
what he said was mostly bull
Synonyms: bullshit / Irish bull / horseshit / shit / crap / dogshit
bull (n.)
a serious and ludicrous blunder;
he made a bad bull of the assignment
bull (n.)
uncomplimentary terms for a policeman;
Synonyms: cop / copper / fuzz / pig
bull (n.)
an investor with an optimistic market outlook; an investor who expects prices to rise and so buys now for resale later;
bull (n.)
the center of a target;
Synonyms: bull's eye
bull (n.)
a formal proclamation issued by the pope (usually written in antiquated characters and sealed with a leaden bulla);
Synonyms: papal bull
bull (n.)
mature male of various mammals of which the female is called `cow'; e.g. whales or elephants or especially cattle;
2
bull (v.)
push or force;
He bulled through his demands
Synonyms: bull through
bull (v.)
try to raise the price of stocks through speculative buying;
bull (v.)
speak insincerely or without regard for facts or truths;
Synonyms: talk through one's hat / bullshit / fake
bull (v.)
advance in price;
stocks were bulling
3
Bull (n.)
(astrology) a person who is born while the sun is in Taurus;
Synonyms: Taurus
Bull (n.)
the second sign of the zodiac; the sun is in this sign from about April 20 to May 20;
Synonyms: Taurus / Taurus the Bull
From wordnet.princeton.edu