The difference between Decent and Underdressed

When used as adjectives, decent means appropriate, whereas underdressed means insufficiently dressed, as for cold weather.

check bellow for the other definitions of Decent and Underdressed

Decent as an adjective (obsolete):

Appropriate; suitable for the circumstances.
Decent as an adjective (of a person):

Having a suitable conformity to basic moral standards; showing integrity, fairness, or other characteristics associated with moral uprightness.
Decent as an adjective (informal):

Sufficiently clothed or dressed to be seen.

Examples:

"Are you decent? May I come in?"
Decent as an adjective:

Fair; good enough; okay.

Examples:

"He's a decent saxophonist, but probably not good enough to make a career of it."
Decent as an adjective:

Significant; substantial.

Examples:

"There are a decent number of references out there, if you can find them."
Decent as an adjective (obsolete):

Comely; shapely; well-formed.

Examples:

"A sable stole of cyprus lawn / Over thy decent shoulders drawn"

Underdressed as an adjective:

Insufficiently dressed, as for cold weather.
Underdressed as an adjective:

Insufficiently well-dressed, as for a formal event.