A motto of the Scottish House of Gordon. It is generally accepted to mean 'abiding'. (There are two mottos for the house, the other being Animo non astutia.)
There are two theories of its origin:
Theory #1: It is a corruption of a Gaelic term. The exact term is uncertain, and, as such, the translations vary considerably. It is usually interpreted as steadfast, but also comes across as enduring, faithful, and abiding. There are further linguistic studies that indicate the more accurate meaning would be 'to stand prepared.' The implication then is that one has been and currently is standing ready, fully prepared for whatever might come. In this sense, it corresponds very closely to the United States Marine Corps motto, Sempre Fidelis.
Theory #2: Bydand is a shortened version of the Scots phrase "Bide and Fecht" meaning to stand and fight. Over the years, it came to be understood that if one were to 'bide,' one was certainly going to 'fecht.'