Why is it der Offenbacher?

Offenbacher ends in -er, and most nouns ending in -er are masculine: in a German dictionary, 86% of them take der.

All nouns ending in -er
Sense 1
der

Meaning

[1] Residents of Offenbach, someone who comes from Offenbach

[1] Bewohner von Offenbach, jemand der aus Offenbach stammt

Examples

[1] My neighbor is a native of Offenbach

[1] Mein Wohnungsnachbar ist ein gebürtiger Offenbacher.

Declension

1 Singular Plural
Nominativ der Offenbacher die Offenbacher
Genitiv des Offenbachers der Offenbacher
Dativ dem Offenbacher den Offenbachern
Akkusativ den Offenbacher die Offenbacher

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary