McDonald & MacNewcastle are Scottish names, but what does the Mc & Mac stand for?

The word is 'SCOTTISH', Sir. 
[--You insult the clans with your careless mis-spelling].
The MacDonalds are a very proud SCOTTISH Highland clan family and bear no connection whatsoever with the rubbish-food American franchise name.
As for 'MacNewcastle', this is a bit of tomfoolery because there is a city called Newcastle, but no such SCOTTISH clan.
[---Back to school, now!!]

The "Mac" in many Scottish names stands for "son of"....

These McDonalds and sons of these Mc's Macs had a Bryan McDonald, who settled in New Castle, Del. 1696 and these wee 'macs' went all over the world, settling in many parts of it. The first McDonald to register a trade name, did it on the 4th May, 1961 - a Drive-In Restaurant and well along with some of my old ancesters, a different Mc a MacDonald, settled in New Zealand and produced wines.

It's a Gaelic prefix for "Son of" used in Scottish and Irish genealogy and surnames.