diminuecer

A rare, nonstandard verb meaning to diminish or lessen, overlapping functionally with the standard disminuir — likely a regional or dialectal blend that never became part of standard Spanish.

• Con la edad, sus fuerzas diminuecieron poco a poco. — With age, his strength diminished little by little.
• Es un verbo raro; la mayoría diría que las fuerzas disminuyeron, no que diminuecieron. — It's a rare verb; most people would say the strength decreased (disminuyó), not 'diminueció'.
• Diminuecer casi no aparece fuera de glosarios dialectales. — Diminuecer barely appears outside dialectal glossaries.

Using diminuecer instead of the standard disminuir — diminuecer is a rare, nonstandard variant; disminuir is what every native speaker actually uses and what a learner should default to.

Often confused with disminuir.