On the other hand, I was struck by how many of you responded in comments to say that you avoid this word entirely due to confusion over which is the correct pronunciation. Word avoidance based on pronunciation ambiguity is an interesting subject to which I haven't seen much attention paid in contemporary linguistics. It's arisen a couple of times over the history of this series of polls, but never so definitively as with lasso. Fortunately, the synonym lariat is ready at hand - itself also from American Spanish, a reanalysis of la reata 'the lariat', which in turn comes from the verb reatar 'to tie again'. So, although there is a definite and unambiguous dialect difference between British and American Englishes, enough of you are aware of the variants that you avoid the word entirely.
The OED, in what I consider to be an unusually high level of snarkiness, offers this pronunciation commentary in its definition: Fowler remarked (Mod. Eng. Usage, 1926, p. 315) ‘lasso is pronounced lasoo´ by those who use it; but the English pronunciation is la'so.’ In ed. 2 (1965) Sir E. Gowers changed this to ‘lasso is pronounced lasoo´ by those who use it, and by most English people too’.] (Fowler in this case is Henry W. Fowler, the British English grammarian whose Dictionary of Modern English Usage remains a standard manual for many British English speakers, especially those who are linguistically conservative). So in 1926, we can be certain, a large number of British English speakers said /læso/ or something quite like it (Fowler's transcription isn't precise) while by 1965, Gowers' second edition reflected a new consensus in Britain, which remains in force today, but which is not longstanding.
On the American side, the American Heritage Dictionary offers only a terse comment on the issue, but it raises a perplexity, because it suggests that /læsu/ is the *older* of the two and that /læso/ is the newcomer. Clearly the comment is correct that Americans mostly say /læso/, but how then to explain the British situation as described in Fowler?
If we presume that both Fowler and the AHD have their facts right, then the most likely scenario, I think, goes like this:
1) The word lazo is borrowed from Spanish into American English in the context of cultural contact in the US Southwest. This is confirmed by the earliest quotations from the OED.
2) For reasons that remain unclear, Spanish /laso/ becomes American English /læsu/. The change in the first vowel is expected given the phonology of the two languages, but the final vowel shift is inexplicable.
3) The word enters more general parlance in both America and the UK, but in doing so is transformed to /læso/ in both Britain and America, based on the orthography. Most people would expect that lasso should rhyme with basso and Picasso, on that basis alone. However, actual users of lassos (presumably mostly Americans whose occupations relate to cattle and horses), continue to use /læsu/.
4) In the mid-20th century, for reasons that remain unclear, British speakers revert to /læsu/. Why? My best guess is that it is due to the pervasive influence of Fowler himself. As a highly prescriptive language usage manual that enjoyed tremendous popularity, it might be that Fowler's statement brought about a linguistic change back to the older form. Fowler was not, however, the manual of choice in the United States, so the change didn't happen there.
On top of the difference in vowel quality there is also variation the stress pattern, which I didn't ask about in the poll. Most British 'lass-oo' speakers pronounce it with second-syllable stress, while most American English speakers are just the opposite - I've heard LASS-oh and lass-OO commonly, and LASS-oo more infrequently, but never lass-OH to my recollection. The stress distinction is interesting because, I suspect, if one were to ask how one pronounces the verb 'lassoed', many straight up-and-down 'LASS-oh' speakers from the US would nonetheless say lass-SOOED or something of that sort. Just a guess, though.
This is all pretty speculative stuff, and the various factors that may have contributed to the rising and falling fortunes of these two variants may never be entirely clear. Language is an ever-changing and slippery beast, and no matter how much we may wish to harness it with a bit and bridle, it is, and will always remain, a bronco that evades our most accurately thrown ... lariat.
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