I would think it came from m'aidez, not m'aider. Aidez is second person plural, as in "you--any of you--help me"; aider is 'to help'. 'To help me' doesn't make a whole lot of sense, if that is all that is said.

Erm-- as a child I "learned" from watching 101 Dalmatians (the '61 version) that SOS means Save Our Skins!