We found one dictionary that defines the word cancellation dumb:

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1. Glossary of Stamp Collecting Terms (No longer online)

▸ Words similar to cancellation dumb ▸ Usage examples for cancellation dumb ▸ Idioms related to cancellation dumb ▸ Wikipedia articles (New!) ▸ Words that often appear near cancellation dumb ▸ Rhymes of cancellation dumb ▸ Invented words related to cancellation dumb