Labdacus was the son of Polydorus (Cadmus' son) and Nyctei's. Robert Graves rather contrivedly offers lampadon acos as an origin for the name, this meaning 'help of torches' and referring to some torchlight procession heralding a symbolic ruler.

Apollodorus, however, makes a rather neat suggestion: Labdacus, he says, starts with the Greek letter lambda (also called labda) (λ), and the name resembles the word for the letter; this letter follows the Greek letter kappa (K) in the alphabet, and kappa is the first letter of Cadmus' name! Alas, Labdacus' own son did not have a name that began with the next letter of the alphabet, mu (M) but was Laïus, the first husband of Jocasta and the father of Oedipus.