English feminine form of Henry [q.v.], from Old German "head of household", a parallel derivation to that of the French name Henriette.
Henrietta has been used as an Anglicized form of the Gaelic Oighrig.
In the 1841 Census of Aberdeenshire there are several spelling variations, the most numerous of which were Hendrietta, a feminine version of the Scottish Hendry, and Heneretta. In general all the variations should be found with a Soundex search on Henrietta and on Hendrietta.
The shortened spelling Henriet has been found in Arradoul Baptismal Records in Rathven parish, in Banffshire, Scotland in the 18th century by a correspondent [GB].