Common words:
* under: beneath, below
* understand: to comprehend, grasp the meaning of
* underneath: below, beneath
* undecided: not having made a decision
* underline: to emphasize, draw a line under
Less common words:
* undergo: to experience, endure
* underestimate: to judge something as being less than it actually is
* underdeveloped: not fully grown or developed
* undemanding: not requiring much effort
* undeniably: certainly, unquestionably
* undertake: to begin or attempt
* undue: excessive, improper
* undulate: to move in a wavelike motion
* undressed: not wearing clothes
* underworld: the world of crime and criminals
Words with specialized meanings:
* undependable: unreliable
* undeterred: not discouraged
* underwrite: to guarantee financially
* underemployed: working at a job below one's skill level
* underfoot: beneath one's feet
Let me know if you'd like more words or if you're looking for something specific!