under

understand, undertake, undergo, underway

prefix under- (too little, below, under):
underneath, underside, underlie, underscore, underline, undersign, underpin, undersea, underwater, underground, undercurrent, undertow, undercover, understate, understatement, underestimate, underrate, underdeveloped, undertone, underarm, underbelly, underfoot, underhand, undermine, undercut, underbid, underperform, underwhelmed, underemployed, underpaid, underpay, underdog, underclass, underprivileged, underserved, underappreciated, undermotivated, understudy, undermount, undersink, underclothes, undershorts, underwear, undershirt, underdress, undercoat, undersize, underage, underweight, undergrown, underfeed, undercook, underdo, underdone, underachieve, undercharge, underprice, undersell, undersold, undervalue, underbrush, underexpose, underemphasize, underutilize, underuse, underrepresent, underfunded, underinvest, underinsure, understaff, undersupply, underdeliver, underpowered, underinflated, undergraduate (undergrad)