international radio-telephone distress call, 1923, apparently an Englished spelling of French m'aider, shortening of venez m'aider "come help me!" But possibly a random coinage with coincidental resemblance:

"May Day" Is Airplane SOS

ENGLISH aviators who use radio telephone transmitting sets on their planes, instead of telegraph sets, have been puzzling over the problem of choosing a distress call for transmission by voice. The letters SOS wouldn't do, and just plain "help!" was not liked, and so "May Day" was chosen. This was thought particularly fitting since it sounds very much like the French m'aidez, which means "help me." ["The Wireless Age," June 1923]