ETYM Old Fren. bastard, bastart, French bâtard, prob. from Old Fren. bast, French bât, a packsaddle used as a bed by the muleteers (fr. Late Lat. bastum) + -ard.
1. The illegitimate offspring of unmarried parents; SYN. by-blow, love child, illegitimate child, illegitimate.
2. A pejorative term for a cruel, base person.

(Vulgar Slang) asshole, jerk, stupid person

Bastard — sometimes used interjectionally to express surprise or disappointment