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Explanation: Looking in most good dictionaries one can find "cu." as cubic ("cúbico") and m. as metre (metro). One would assume therefore, as another person already has done, that cu.m. = metro cúbico.
I've just looked on the Eurodictautom site and there are some other references to "cu." but not with the "m." term. See the ref. below.
I hope this helps.
Neil (English prof).