Why is it der Slipper?

Slipper 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] Flat shoes

[1] flacher Halbschuh

Examples

[1] "Negusses feet, which, as with every pubescent boy, look far too big for the lanky body, are in cheap, dirty, artificial ledern slippert" without socks "

[1] „Negusses Füße, die wie bei jedem pubertierenden Jungen viel zu groß für den schlaksigen Körper wirken, stecken ohne Socken in billigen, schmutzigen, kunstledernen Slippern.“

[1] "The slippers clap while walking against their feet"

[1] „Die Slipper klatschen beim Gehen gegen ihre Fußsohlen.“
More source examples (1)

These dictionary examples can include historical and less common uses.

[1] "His slippers have been removed, but he is more than it seems that this man"

[1] „Seine Slipper sind abgetragen, aber er ist mehr als er scheint, dieser Mann.“

Declension

1 Singular Plural
Nominativ der Slipper die Slipper
Genitiv des Slippers der Slipper
Dativ dem Slipper den Slippern
Akkusativ den Slipper die Slipper

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary