'Emmet' is actually Old English, rather than Cornish. The word was adopted by the Cornish during the 'changeover' from Cornish to modern day English. It literally means 'ant'. The Cornish word for 'ant' is Muryonen, or Muryon (plural).
Originally 'Emmet' referred to the hordes of money-grabbing English speakers who appeared in Cornwall when copper reserves were discovered there. The way in which they swarmed over the Duchy seemed to resemble ants.
Nowadays the word is used to refer to the hordes of holidaymakers who swarm the county in summer. Usually these tourists are sunburnt, overweight, carrying polystyrene 'surfboards', and drink lots of Wife-beater.
'Emmet' is a particularly effective word for describing tourists: they really do trail like ants, all heading for the same few spots, all summer long. Most tourists only have two legs though.
Disclaimer: Not all tourists visiting Cornwall are overweight, sunburnt lager-louts. Those with more money purchase luxury apartments and quaint cottages and frequent The Eden Project. These tend to drink Chardonnay, not Stella.