dated: love child, by-blow; natural child/son/daughter.
The immense popularity of Game of Thrones has put the repugnant term back into the modern lexicon. Not surprising given the very real threat illegitimate spawn posed to medieval power brokers.
All major religions in the Seven Kingdoms – the Faith of the Seven, followers of the Old Gods of the Forest, and followers of the Drowned God – attach very negative social stigmas to bastardy. ¹
synonyms: scoundrel, villain, rogue, rascal, weasel, snake, snake in the grass, miscreant, good-for-nothing, reprobate;
informal lowlife, creep, nogoodnik, scamp, scalawag, jerk, beast, rat, ratfink, louse, swine, dog, skunk, heel; slimeball, son of a bitch, SOB, scumbag, scumbucket, scuzzball, scuzzbag, dirtbag, sleazeball, sleazebag;
dated hound, cad;
archaic blackguard, knave, varlet, whoreson