The etymology of the first part this term is obscure, there are obvious reasons why it may have been suppressed or forgotten. It probably derives from various Norse or Scandinavian words for 'knot', such as cnut. Harthacnut, the name of a decendant of King Canute or Cnut the Great, means 'tough knot' (compare Middle Dutch cnudde, from Proto-Germanic *knuttan-; cognate with knod, English knot, Frisian knotte, Danish knude, Norwegian knute, Swedish knut, etc).
A cut splice is essentially splicing a cut rope back together with a knot, and the term may have been compounded or corrupted by British sailors from a phrase such as 'cut-knot splice'. Or, as it is formed with the butts of two ropes, and often performed on a wooden block called a butt or a bitt, the term 'butt-splice' may have been vulgarised or associated with a Dutch term for the technique, as the Dutch cont can mean butt, bottom, vagina.