Licentiousness means "without restraint, excess" and it refers to anything that is controlling people to do in excess - sometimes used in connection with sexual sins but can be used for things like over-eating...etc.
We need to read the verse before this one here to see exactly want solid food is - it's the word of righteousness.
The author is saying that those that partake of the milk is not accustomed to the "word of righteousness". Solid food is knowing the word of righteousness. Hebrews 5:13-14
Hebrews 5:13 (NASB)
[SUP]13 [/SUP] For everyone who partakes only of milk is not accustomed to the word of righteousness, for he is an infant.
Not accustomedmeans "unacquainted with something." ( Complete Biblical Library Greek - English Dictionary )
Those trying to create and maintain their own righteousness by what they do or don't do - are still spiritual babies. They are not established in the "gift of righteousness" yet - which only comes by grace through faith in what Christ has already done. That He alone is our righteousness.
The Jews were notorious for trying to establish their own righteousness by the Law of Moses. Romans 10:1-5 - Remember this is the book written to Hebrews.
Heb. 5:14 - But solid food ( word of righteousness ) is for the mature, who because of practice have their senses trained to discern good and evil ( Greek word for evil = bad, useless, worthless ).
The mature person in Christ knows when they are practicing the deeds "of His righteousness" which is the good - or doing deeds of righteousness "to be righteous" which are bad, evil, useless or worthless. This is the evil or useless side of Heb. 5:14