Definition
"Er of" does not appear to be a recognized legal term. The entry as submitted is likely a fragment, truncation, or scanning artifact — most probably the tail end of a longer compound term such as "Gentleman Usher of the Black Rod," "Bearer of," "Keeper of," or another officer-of-the-crown formula common in English legal and parliamentary usage.
The Black's Law Dictionary entry associated with this fragment describes a chief officer of the English Crown identified by a ceremonial black rod topped with a golden lion — a description that matches the Gentleman Usher of the Black Rod, a senior officer of the House of Lords. That officer's role includes ceremonial functions in Parliament, service of summons, and enforcement of the orders of the House of Lords.
If the intended term is "Usher of the Black Rod" or "Gentleman Usher of the Black Rod," that entry would properly describe a parliamentary and royal household officer in England whose office dates to the reign of Edward III. The title derives from the ebony rod of office carried as the symbol of authority.
Why It Matters in Research
Researchers encountering "er of" in a historical legal source should treat it as a probable truncation. Optical character recognition errors, damaged print margins, and abridged index entries frequently strip the leading syllables from compound officer titles in English legal materials. The fragment "-er of" is consistent with dozens of officer designations: Keeper of the Great Seal, Bearer of the Rod, Master of the Rolls, Speaker of the House, and similar formulations.
When working with pre-twentieth-century English legal dictionaries or parliamentary records in digitized form, search for the full officer title rather than the fragment. Black's Law Dictionary entries on English Crown and parliamentary officers are often brief and should be supplemented with Halsbury's Laws of England or parliamentary reference works for substantive content.
If the research context is American, note that most English Crown officer titles of this type have no domestic American equivalent and appear in U.S. legal sources only in historical or comparative discussions of parliamentary procedure.
Historical Dictionary Support
Black's Law Dictionary supplies only the fragment and the associated description of a chief Crown officer carrying a ceremonial black rod. This matches the Gentleman Usher of the Black Rod without naming the office in full — consistent with the probability that the entry as captured here is incomplete. Historical legal dictionaries of the eighteenth and nineteenth centuries, including Bouvier's and Tomlin's, carried entries for Black Rod and related parliamentary officers under their full titles. No independent legal meaning attaches to the string "er of" standing alone.