bastard

when you mean someone whose parents were not married at the time of their birth (and you really, really need to mention this), use "offspring/child/son/daughter of unmarried parents/of single parent/of unknown father." When you mean "bastard" in the sense of "a wretched and repellant male" (Richard A. Spears, NTC's Dictionary of American Slang and Colloquial Expressions), you'll have to be creative—most alternatives are unacceptable on grounds of sexism, handicapism, ageism, or unprintable language. Try ignoramus, saphead, stinker, ratfink, snake in the grass, creep, heel, jerk, bum, lowlife. "Bastard" is sexist because it is used only for men and because the insult also slyly impugns the man's mother. See also bad guy, illegitimate/illegitimate child.