The phrase "wast slothful" is an old-fashioned way of saying "you were lazy". The spelling might seem strange to modern readers, but it reflects the way English was pronounced in the past. "Wast" is the second person singular past tense of "be", and is pronounced /wɑst/. "Slothful" means lazy or idle, and is pronounced /ˈslɑθfəl/. Together, the two words convey a stern admonition to avoid laziness and work diligently. While this phrasing may no longer be commonly used, it remains a reminder of the evolution of the English language over time.
The phrase "wast slothful" appears to be a combination of words from different origins.
1. "Wast" is the archaic second-person singular form of the verb "be" in the past tense. The word "be" has Old English and Proto-Germanic roots, ultimately deriving from Proto-Indo-European.
2. "Slothful" is an adjective that means lazy or indolent. It comes from the Middle English word "slouthe", which originated from Old English "slǣw", meaning slow or sluggish. "Slǣw" can be traced back to the Proto-Germanic word "*slawaz" and the Proto-Indo-European root "*slowo-".