Bite Off More Than You Can Chew

Rooted in genuine 19th-century American habits around chewing tobacco, which was commonly sold in solid pressed blocks or "cakes." Someone greedily biting off a chunk larger than they could comfortably chew created an obvious, relatable image of over-ambition, first appearing in print in American publications during the 1830s. The metaphorical sense, taking on more than you can actually manage in any context, followed naturally and needed no real updating over the following century and a half.