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The first M is Italian for "mille"
Which in turn came from the old Latin, hence the similar use of M for 1000 in Roman numerals (e.g. MMIV = 2004).

It crops up in other fields as well, e.g. in property tax assessments where the "millage" rate is applied as an amount per thousand.

Mike,
The U.K. has used sq. millimeters for wire sizes since 1970, but prior to that we had SWG (Standard Wire Gauge) which works in a similar way to AWG, i.e. bigger number = smaller wire.

House wiring cables however, were generally listed by number of strands and size, e.g. a common size was 7/.029 meaning seven strands, each 0.029" diameter.

I'm not aware of circular mils ever being used to designate conductor sizes here.