"emmet-butt" - Westcountry dialect

Emmet is an old word for an ant, and Butt (or Butte) is a hill, in both cases not just in the west country dialect, so you are right to think that an emmet butt is an anthill.

In the west country a batch is a low hill, there are many place names in Somerset for example which incorporate it. However, batchy is an adjective meaning stupid.

You should also consider that there is more than one west country dialect. Words are not necessarily the same as you move further west. I would suspect both are correct in different counties. Unfortunately I can't be more specific as, although I'm Somerset born and bred, I can't say I remember hearing either used.

According to the OED, emmet is ultimately a variant of ant and derives from the Old English word æmete:

Emmet ... {repr. OE æmete wk. fem. (see ANT). The OE æ in stressed initial syllables frequently underwent shortening in ME, and was in that case variously represented according to dialects by ă or ě. Hence the two forms ămete and ěmete ; the former of which became contracted into amt, ANT, while the latter retained its middle vowel and survives as emmet.} 1. A synonym of ANT. Chiefly dial., but often used poet. or arch. Horse-emmet, the Wood Ant (Formica rufa). 2. attrib., as emmet-swarm. Also emmet-batch, -but, -cast (dial.) = ANT-HILL.

The earliest occurrence of emmet that the OED cites is from the circa 850 Kentish Glossary, where it appears as emetan. The earliest match for emmet in an Early English Books Online search is from Biblia the Byble, that is, the holy Scrypture of the Olde and New Testament, faithfully translated in to Englyshe (1535) in a familiar proverb of Solomon:

Go to the Emmet (thou slogarde) cōsidre hir wayes, & lerne to be wyse / She hath no gyde, no teacher, no leder: yet in the sommer she prouideth hir meate, & gathereth hir foode together ī ye haruest.

As for butt (or but), an interesting discussion of this term appears in Ernest Adams, "On the Names of Ants, Earwigs, and Beetles," in Transactions of the Philological Society (1858):

In Somersetshire the ant-hill is called an emmet-batch. Batch is another form of botch, a boil or swelling, and the ant-hill may be compared to a boil or tumour on the face of the field; compare Dutch butse, "Batch: a mound; an open space by the road-side; a sand-bank or patch of ground lying near a river" ([Thomas] Wright). Cf. allern-batch, a kind of botch or old sore (Exmoor). In the same county the ant-hill is also known as an emmet-but. In the dialects of Somerset and Exmoor a bee-hive is termed a bee-but, and Halliwell adds another meaning of the word but: " a peculiar kind of conical basket used in the river Parret for catching salmon".—It is apparently the A.S. word bót or bút, seen in the diminutive bótle or bútle, a house, and still retained in local names in many parts of the kingdom: Bot-ley, But-leigh, Bat-ly, Bet-ley, and Boot-le, and in the modern English booth. 'Emmet-but' may fairly be translated 'ant-house'.

James Jennings, Observations on Some of the Dialect of the West of England, Particularly Somersetshire (1825) offers further details about but in this sense:

But. s. A conical and peculiar kind of basket or trap used in large numbers for catching salmon in the river Parret. The term but, would seem to be a generic one, the actual meaning of which I do not know : it implies, however, some containing vessel or utensil See BEE-BUT [noted earlier in the glossary in this brief entry: "Bee-but. [or] Bee-lippen. s. A bee-hive."]. But, applied to beef, always means buttock.

As noted by the poster of the original question above, Jennings also has an entry for emmet-batch:

Emmet-batch, s. An ant-hill.

But Jennings does not mention emmet-but as an alternative term for "anthill."

One of the earliest instances in print of emmet-but (cited by Adams in his 1858 article) appears in James Halliwell-Phillipps, A Dictionary of Archaic and Provincial Words, Obsolete Phrases, Proverbs, and Ancient Customs, from the Fourteenth Century (1847):

EMMET-BATCH. An ant-hill. Somerset. Also called an emmet-but.

Even earlier, however, is this mention of emmet-butt in the glossary of William Barnes, Poems of Rural Life, in the Dorset Dialect, with a Glossary (1844):

Emmet-butt or Emmet-hill. An ant hill.

Barnes uses the term emmet buts in his poem "The Common A-Took In," printed in that same volume:

Ees, while the ragged colts did crop / The nibbled grass, I used to hop / The emmet buts from top to top / So proud of my spry jumps; / An' thee behine ar at my zide, / Di'st skip so litty an' so wide / 'S thy little frock wo'd let thee stride / Among the vuzzy humps.

Barnes returns to the topic of emmet butts in "The Waggon A-Stooded," in Hwomely Rhymes: A Second Collection of Poems in the Dorset Dialect (1859):

What then. I coulden leäve the beäten track, / To turn the waggon awver on the back / Ov oone o' theäsem emmet-butts. / If you be sich a drever, an' do know 't, / You dreve the plow, then ; but you'll awverdrow 't.

...

Oh! ees, d'ye think I be a nincompoop, / No, no. The lwoad wer built so firm 's a rock, / But two o' theäsem emmet-butts would knock / The tightest barrel out o' hoop.

It thus appears that Dorset has as good a claim as Somerset to early published use of emmet butt.

I be from Templecombe,I be 76 gwoin on 77, an ant hill was always called an emmit butt. If volk sed ‘ant hill’ they was posh, I knowd somebody once oo always sed that, bugger, I always took me ‘at off to in