This was the Spanish-Portuguese rendering of Elizabeth, via Isabeau & Ilsabeth and probably came to Scotland and Britain via the French Isabelle.
NB It has been a long tradition in Scotland for Elizabeth and Isobel [and their variants, pet names, etc] to be used interchangeably, i.e. as synonyms. This can be confusing when searching through older records and care should be taken to consider this possibility when researching.
The 1841 Census of Aberdeenshire has a large number of variations in the spelling, almost all of which will be found using Soundex searching. However many of the abbreviated forms may not be found by Soundex; we have listed these above among the diminutives. The most frequent spellings are shown here under Isobel unless it is deemed to be an obvious variant of Isabel [q.v.].
A correspondent [BL] has found the spelling Isobele on a gravestone in Auchindoir, Aberdeenshire, Scotland. This should also be found by Soundex searching.