Berserk, Pea, and Edit

A back-formation is a word created by removing an element from an existing word. It can change the word’s meaning or the part of speech.

Berserk  is a back-formation from berserker, one of a band of ancient Norse warriors legendary for their savagery and reckless frenzy in battle.

Pea and plural peas are a back-formation of Middle English 'pease', which was a mass noun in wide use, like oatmeal.

Edit is a back formation of editor. Editor was in use for almost 150 years before edit. Edit arose because the word 'editor' sounds as if it should mean 'one who edits'.