I'm fascinated by etymology & toponymy, and try as far as is reasonably possible to discover the origins and meanings of Cumbrian "dialect" words.
I'm aware of the "GoneMad Cumbrian Dictionary and Phrasebook",
and other sources of reference such as: -
"The Cumbrian Dictionary of Dialect, Tradition and Folklore by William Rollinson" and "Lakeland Words - (a collection of Dialect Words and Phrases as used in Cumberland and Westmorland) - B. Kirkby.
Unfortunately, non of the above provide the meaning of the word "Mart", in the context of Martcrag Moor.
I know that Mart can be taken as the origin of Market, as in "Cattle Mart", and I recall reading somewhere that Mart is an old Irish word for the carcass of an Ox or Cow that's been slaughtered for meat.
Could it be - in a similar context to "Gimmer Crag" - where Gimmer means yearling sheep, (i.e. the crag where yearling sheep are kept), that Martcrag Moor originally meant the moor where Ox or Cows are (or were once) kept?
If anyone can give a definitive answer to the above, I'd be grateful. - And if anyone has additional/useful sources for the meanings of Lakeland dialect words, I'd be interested to hear what they are.
As a post-script to the above, I've just obtained a copy of the book "A Dictionary of Lake District Place-Names" (2006), by Diana Whaley, which gives the origin of "Mart Crag" as being "the rocky height frequented by Pine Martins".
This would appear to be the most likely origin of the place-name, since, as the book goes on to explain, Pine Martins are/were locally called "Marts" - that word being derived from an Old English word (similar to "meard" - but spelled using characters that aren't within the available character set for posting here).
The above mentioned book is from the "Regional Series", published by the English Place-Name Society, and is apparently regarded as the authoritative work on the subject.
For anyone interested in toponymy, I'd highly recommend it. - It's one of those books that can be browsed at leisure, and opening it at almost any page will probably give you some insight to a Lake District place-name that you weren't previously aware of.
Another, smaller book I've just acquired on the same subject is "Lake District Place Names" (revised edition 2013), by Robert Gambles. - Just as fascinating, and perhaps easier to obtain, but not quite as extensive in its content.