Where does “slappeth” come from?

slappeth (English) comes from English slap, from Middle English slappen.

slappeth (English): Third-person singular simple present indicative...

Definitions

  1. Third-person singular simple present indicative...

Ancestry of “slappeth”, step by step

slappeth traces back through two separate lines of ancestry.

via English slap

StepLanguageWordMeaning
1EnglishslapA blow, especially one given with the open hand,...
2Middle Englishslappen

via English eth

StepLanguageWordMeaning
1EnglishethA letter introduced into Old English to...
Every word from Middle English slappenEvery word from English slap