The Oxford Names Companion, OUP "Scottish Forenames" - Donald Whyte, FGH, FSG
Notes:
Scottish Gaelic pet name of or a variant of Elisaid/Elasaid/Ealasaid, the Gaelic equivalents of Elizabeth, from an ancient Hebrew name meaning "oath of God". As a pet name Beitidh is a variant of Betty and Betsy.
Thanks to Donald Whyte who provided much of the initial information for this site.