"Wert on side" is a colloquial phrase used to describe someone who is disloyal or unfaithful. The spelling of the word "wert" is actually a variant of the past tense of the verb "be." It is pronounced /wɜːrt/, with a long "u" sound and a soft "r." The phrase itself is believed to originate from old English, where "wert" was commonly used as the past tense of "be." Today, the phrase is often used in casual conversation to describe someone who has betrayed a friend or ally.