" unpremeditated "

1: happening by chance;

  * e.g., " ... an unpremeditated encounter with an old boyfriend "

•  Synonyms : 

•  accidental,

•  casual,

•  chance,

•  fluky,

•  fortuitous,

•  inadvertent,

•  incidental,

•  unintended,

•  unintentional,

•  unplanned,

•  unwitting

•  Antonyms : 

•  calculated,

•  deliberate,

•  intended,

•  intentional,

•  planned,

•  premeditated,

•  premeditative,

•  prepense,

•  set

2: made or done without previous thought or preparation;

  * e.g., " ... an unpremeditated verbal attack that was prompted by an unexpected question at the press conference "

•  Synonyms : 

•  ad hoc,

•  ad-lib,

•  down and dirty,

•  extemporaneous,

•  extemporary,

•  extempore,

•  impromptu,

•  improvisational,

•  improvised,

•  off-the-cuff,

•  offhand,

•  offhanded,

•  snap,

•  spur-of-the-moment,

•  unconsidered,

•  unplanned,

•  unprepared,

•  unrehearsed,

•  unstudied

•  Antonyms : 

•  considered,

•  planned,

•  premeditated,

•  premeditative,

•  prepared,

•  rehearsed