English, from Old German "peace ruler".
There are a few spelling variations for Frederick in the 1841 Census of Aberdeenshire which should all be found in most databases by using Soundex searching.
A personal correspondent has found the pet names Rick and Ricky/Rickie used for Frederick although we believe these are normally used for Richard.
Old documents in Latin used Fredericus (and possibly Fridericus) for Frederick, with appropriate Latin case endings.
The Irish Gaelic forename Feardorcha can be Anglicized as Frederick or Ferdinand so these latter two names are classed here as a lesser synonyms.