E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Properly the door-sill, but figuratively applied to the beginning of anything; as, the threshold of life (infancy), the threshold of an argument (the commencement), the threshold of the inquiry (the first part of the investigation). (Saxon, thrscwald, door-wood; German, thürschwelle; Icelandic, throsulldur. From thür comes our door.)