случайно

1) General subject: accidentally, as it happens, as luck would have it, at haphazard, at large, at-large, by accident, by chance, by haphazard, by haphazard, casually, circumstantially, coincidentally, extrinsically, facultatively, haphazard, haphazardly, haply, happy go lucky, in a promiscuous manner, inadvertently, incidentally, mistakenly, obiter, promiscuously, sporadically, stumble (на что-л.), transiently, by mistake, by any chance, anecdotally, unwittingly

2) Colloquial: promiscuous

3) American: on accident

4) Obsolete: perchance

5) Bookish: fortuitously

6) Mathematics: at random, occasionally, per accidens, randomly, stochastically

7) Psychology: circumstantiality

8) Astronautics: incidental

9) Advertising: on a chance basis

10) Business: on occasion

11) Oceanography: ad hoc

12) Makarov: happy-go-lucky