My understanding is that "over-" and "under-" expose always refers to the light meter indications. People would normally say: "in order to take a picture of a black cat with some coal as background, underexpose by 1.5 EV or so" and that would imply "compared to the exposure suggested by your light meter".
The meaning the OP is referring to, that is over- and underexposing beyond intention and scope, is normally described as "mistakenly overexposed", "grossly overexposed", or some words to that effect.
It's easier to stick to the abovementioned convention, rather than saying any time: "in order to take a picture of a black cat with some coal as background, take the reading of your lightmeter and increase exposure by 1.5 stops".
That's what I see in that word.
Fabrizio
PS I suppose life is easier for gardeners