'motherboard' etymology

[Jules Richardson]

Does anyone know the origins of the term 'motherboard'?

I've always associated it with computers and assumed that it started 
appearing somewhere around 1980, with the fading out of passive backplane 
systems and arrival of machines which put more functionality onto a 'core' 
PCB into which other cards were plugged. I don't recall ever seeing it used 
when referencing earlier big iron, but maybe I've just missed it.

I had the case lid off a Fluke digital multimeter which hails from 1972 
earlier, and was surprised to see it written as part of a warning there 
("ensure that all cards are securely plugged into the motherboard before 
applying power", or similar - unfortunately I didn't grab a photo at the 
time).

cheers

Jules