Pipe-weed

‘It was Tobold Hornblower, of Longbottom in the Southfarthing, who first grew the true pipe-weed in his gardens, about the year 1070 according to our reckoning.’ (LotR in. viii)

Pipe-weed is the hobbits' name for tobacco. It is not a particularly old term, and the OED (Second Edition) illustrates it only from Tolkien, though subsequent research for OED Online has located examples as far back as an American magazine of 1792. Weed on its own has been used to mean 'tobacco' since 1606 (OED: weed n.133). In the earliest draft of the passage quoted above, Merry spoke of weed (as Gimli still does in the published text), but this was replaced by pipe-weed in a subsequent stage of rewriting (HME VI. 36-7). It is a natural-sounding compound of English words (resembling other old plant names ending in -weed), and is clearly more suited to hobbit-speech than the exotic Caribbean loanword tobacco (though this does appear in the more anachronistic text of The Hobbit). The similarly alien word potato sometimes appears in the English colloquial form taters, which helps to disguise the covert anachronism of a New World plant in a supposedly Old World setting (see also Shippers The Road to Middle-earth, pp. 78-9).

The Ring of Words ~
Tolkien and the Oxford English Dictionary
Oxford University Press (2006)