Though that last way of putting it, which shows up in multiple dictionaries, is weakly worded -- it implies an act of will, but the true sense is it cannot be escaped, rather than is not to be. This was borrowed around 1620 from Latin inēluctābilis, from in-, not + ēluctārī, to struggle or force out of, that last being from ē-, a form of ex-, out + luctārī, to wrestle, which is related to lock, as in an arm lock. Yes, this highfalutin' sounding term comes to us from an ancient professional wrestling term. I like that.
He stared at the empty closet, then back at the note again, five more times, before finally coming to the ineluctable truth: Sarah had indeed left him.