M - or K - or G! (8)
I believe the answer is:
thousand
I believe this is a double definition.
'm or k' is the first definition.
(I've seen this before)
'g' is the second definition.
(synonyms)
'or' acts as a link.
(Other definitions for thousand that I've seen before include "in MCC, but missing double century" , "a fair amount?" , "Ten hundred" , "Large number" , "Numeric value of kilo" .)