The Hebrew word בול bul has four meanings:
a) a stamp
b) a lump, a block
c) produce (noun)
d) absolutely correct
Almagor-Ramon in Rega Shel Ivrit writes that there's no relation between them. Klein lists them all as separate terms. But is there any connection?
Where does the term for "stamp" come from? According to Almagor-Ramon, it comes from the same word in Arabic, who borrowed it from Turkish. The Turks borrowed it from Italian, where bolla means "seal". The Italian in turn comes from the Latin bulla, which is still used as the term for a papal edict - originally a "sealed document". This meaning gave rise to the English words bulletin and bill. The original meaning in Latin meant "round thing, knob, bubble" - which referred to the seal itself. From this earlier meaning we get in English bullet, bowling and boil.
The Latin term derived from the Greek bolos (or bulos) - a lump of earth, a rounded mass. This meaning of bul is found in Mishnaic Hebrew (Shabbat 67b) and perhaps even in the Tanach - Yeshayahu 44:19 (although there are those that say it comes from the meaning of "produce".)
While there are those that claim that the Mishnaic bul was borrowed from the Greek, the origin of the Greek word is less certain.
From the discussion here, we see that the American Heritage Dictionary states that the etymology of the Greek bolos is "of obscure origin". (The same discussion explains why the Greek verb for throwing - ballein- is unrelated to bolos.) Klein and Almagor-Ramon both say that the Akkadian bulu - a "block of dry wood" is related. So perhaps this is the source of the Greek bolos?
The meaning of "produce" derives from the Hebrew word yevul יבול meaning "produce, yield, fruit". This is the source of the Biblical name of the month of Cheshvan - ירח בול - yareach bul.
And what of bul meaning "absolutely right" or "straight on"? This one's from English - "bull's eye". How did this expression come about in English? The Word Detective gives the following explanation:
A "bull's-eye" is, of course, the center spot of a target, producing the highest score a shooter (whether in firearms, archery or another competition) can score. "Bull's-eye" is also used to mean a direct hit on such a spot, or, figuratively, the accomplishment of a goal with precision and finality ("Bob scored a bull's-eye on our quarterly sales quotas."). "Bull's-eye" first appeared in this "center of the target" sense around 1833.
The question is, however, why a "bull's-eye"? For the answer, we have to go a bit further back in time. Since the 17th century, "bull's-eye" has been used as a term for almost anything small and circular, especially if it protrudes slightly, forming a hemispherical bump resembling the protruding eye of a bull or cow. Thus, at various times, "bull's-eye" has been used to designate a thick piece of glass set into the deck of a ship to illuminate the lower decks, a one-crown coin of British currency, a globular piece of candy, and a small circular window, among other things. So although the spot in the center of a target doesn't protrude like a real "bull's eye," it is small and circular and thus fit the popular definition of "bull's-eye."
So how did this explanation of bul do? Was I קולע בול - right on target? Or did I come off as a בול עץ - a blockhead?