The person we are beneath our social mask; the feelings and urges we have but may not admit to others or even ourself; the less expressed or capable side of our nature; something we hide.
If dreamer is under something, someone: feeling suppressed, feeling weighed down or worried about something; feeling the underdog.
If dreamer sees something under them: feeling the top dog; seeing one s inferior (not as capable) functions or self; looking down on others; having a wider view. Idioms: come under, down under, under one’s wing; under the weather, under the thumb; under one’s hat, under a cloud; under one’s nose; under the counter, under the influence. See above.
Being underneath something suggests either taking shelter or submitting to someone else’s handling of us. It may also represent the part of us that we hide, or the part that is less capable.