Synonyms and related words :
a propos, accidentally, apropos, apropos of, as it chanced, by a fluke, by accident, by chance, by good fortune, by hazard, by the by, by the way, casually, en passant, for example, for some reason, fortuitously, in passing, in some way, par exemple, parenthetically, perchance, somehow, somehow or other, speaking of, unpredictably