Could be thought of as the grandad-ish version of the name Ethel, by appending the diminutive suffix '-bert' on to that name. However, it was never at all common when the name Ethel was popular, and both names are now out of fashion.
Could be classed as a combination of the once-popular female name Ethel with either Bert or Elbert, or even Albert. Ethelbert has the female version Ethelberta, which is even rarer than Ethelbert, but is borne by a character in Thomas Hardy's 1876 novel 'The Hand Of Ethelberta'.