Although this term can refer to the period before any major flood, it most often means the Noachian flood.  And out of that comes its meaning of extremely old or ancient.  I love the sound of the word itself — the great downward and backward cast of your tongue on the forth syllable has something of a deluge feel to it, I think.  And then, of course, there are wonderful applications such as this example given in the Princeton dictionary:   “a ramshackle antediluvian tenement.”

I would caution writers that this is an adjective very difficult to pull off without sounding pedantic.  If you want my advice, save it for casual conversation such as, “My, that tenement is ramshackle and antediluvian.”