I believe that "mommy/mother on" has nothing at all to do with mothers. During the Second World War, there was an expression that was in fairly common use in the UK, and which was printed on government posters that were put up in public places. The expression was: "Keep calm and carry on," which meant pretty much what it said, i.e. don't let the bombing get you down, but just carry on as normal. This expression was sometimes altered by all manner of people to: "Keep calm and bugger on." Prime Minister Winston Churchill used a similar expression when he ended some of his official messages to Eisenhower or other leaders. The word "Bugger" is not a polite word to use, so it was sometimes changed to something that was more acceptable to delicate ears. It looks as if your expression might have been created out of a desire to use a "nice" word, and, as "mother" could be said to vaguely resemble the b****r word, that's how the expression came to exist. It has nothing to do with mothers; it just means: "Don't lose your nerve, folks, just carry on as normal."