This is a name I've come across up near Scarr & the Bracket Rocks – I thought it was the Lough Spit Stone on account of view of Lough Dan etc. However talking to Jim O'Toole up in Rathdangan, he told me about a old boundary – a mearing up on the mountain in Aughavannagh which was known locally by the same name. He said the word was connected with digging – a spit, and the lock-spit was the next layer down. Sure enough, it seems to be an old English term – one dictionary gives it as 'a small cut with a spade, or a trench opened with a spade or a plow, to mark out a line of work, as in fencing, railway-engineering, or the like.'
So it's the Lock Spit Stone, not the Lough Spit Stone..