"While the exact derivation of the word is not definitively known, the OED posits that it's a combination of lam and baste, both of which bear the sense (now archaic for both words) to beat soundly. Other sources agree. And lambaste is the older form. In historical Google Books searches, lambast is almost nonexistent before 1850. It appears increasingly in the second half of that century, including in works by Charles Dickens and Rudyard Kipling (British authors)."