Pronunciation: [ʌnpˈɔ͡ɪzənəs] (IPA) 
The word "unpoisonous" is spelled with the prefix "un-" added to the word "poisonous". The prefix "un-" means "not", indicating that something is not poisonous. The pronunciation of "unpoisonous" is /ˌʌnpɔɪzənəs/, with the stress on the second syllable. The word is pronounced with a short "u" sound in the first syllable, followed by a stressed "oi" sound in the second syllable, and ending with an unstressed "nəs" sound.
The word "unpoisonous" is formed by combining the prefix "un-" which means "not", and the word "poisonous" which comes from the noun "poison".
The noun "poison" originated in the late 13th century from the Old French word "poison", which was derived from the Latin noun "potio" meaning "a drink" or "a potion". Over time, the term "poison" came to specifically refer to a substance that is harmful or lethal when ingested or absorbed.
By adding the prefix "un-", denoting negation or absence, to "poisonous", the word "unpoisonous" is created to describe something that is not poisonous, i.e., not harmful or toxic.
