what does the term "might as well" mean?

This may be considered General Reference; but it's not easy to find in online dictionaries, so I might as well give it a stab.

macmillandictionary.com, in the ‘Phrases’ under might gives this:

might (just) as well _^INFORMAL
1. used to suggest doing something because you cannot think of anything better to do

We might as well sit down while we're waiting.
You might as well enjoy your money while you've got it.

It indicates an unenthusiastic acquiescence in someone else's proposition, or a less-than-wholehearted proposition of one's own. When I was young, the conversation might run like this:

A. Ya wanna go grab a beer?
B. (shrugging). Might as well. Can't dance, and it's too wet to plow.

The fullest form of the phrase (little heard now) indicates why it has this meaning: might just as well (do X) as not. That is, we're equally well-off doing X as not doing X.

This phrase can be used as an alternative to yes, and a more colourful answer to a question, i.e Would you care for a cup of tea? " Oh, I might as well" meaning what harm could it do! Are you going to take the car or walk? "I might as well take the car". (as walking would involve exerting energy!) It is normally used where there is an alternative, and you declare the winning option by the term I might as well do blah blah etc. Hope this helps. I just thought I might as well try answering it!!

We need to segregate "might" from "as well", first.

Might = probably, possibly.

as well = equivalently appropriate or fitting.

Therefore, "might as well" would mean

possibly just as appropriate or fitting.

For example,

Do you think that Jesus will be returning next year?

He could do just as well by not returning next year, since it has been 2000 years' procrastination.

He might, do just as well, by not returning next year.

He might as well not return at all.

He might be just as comfortable not returning at all, compared to his comfort level of having to fulfill a prophecy - having to deal with a planet damaged by god-fearing people vs the guilt of not fulfilling a prophecy.

In fact, there might be no difference whether he returns or not. The planet would not benefit either way.