The phrase "wast the back of" is not a commonly used one in modern English. However, if we break down the pronunciation using IPA phonetic transcription, we can see that every sound is accounted for. "Wast" is pronounced /wɑːst/ with a long "a" sound and a silent "e". "The" is pronounced /ðə/ with a voiced "th" sound. "Back" is pronounced /bæk/ with a short "a" sound. And finally, "of" is pronounced /əv/ with a schwa sound and voiced "v". Together, the phrase is pronounced: /wɑːst ðə bæk əv/.