underbrush, n. undergrowth, [jungle].
underground, a. subterranean, subterraneous, subterrene, subterrestrial.
underhand, a. [underhanded], surreptitious, [clandestine], [covert].
underhand, adv. clandestinely, covertly, secretly, surreptitiously.
underhanded, a. [covert], [clandestine], surreptitious, [underhand].
underline, v. underscore.
underling, n. subordinate, [inferior], understrapper.
underlying, a. [fundamental], basic.
underrate, v. underestimate, [undervalue], underprize.
understand, v. [comprehend], [apprehend], [grasp], [discern].