"silly "means foolish; not sensible. Examples Don't be so silly! There are some silly children/women/men everywhere. Usually "such a silly" has a noun after the adjective "silly". Example: He is such a silly man[ foolish man] Other examples of slang: He/she is a "silly billy" or a "silly duffer"

"You're such a silly (person)!" is how you'd read it.

It is possible to say 'Don't be such a silly.' In this case, 'silly' is acting as a noun. The phrase means 'Don't be such a foolish person. It can be quite an affectionate statement, between very close friends.