• 熟語じゅくご — idiomatic compound / phrase
• 熟練じゅくれん — skill / mastery
• 未熟みじゅく — immature / inexperienced
• 成熟せいじゅく — maturity
• 熟成じゅくせい — ripening (e.g., cheese / sake)
• 熟考じゅっこう — deep thought
• 熟睡じゅくすい — sound sleep
• 熟読じゅくどく — careful reading

熟 combines the fire radical (灬 / れっか) with an element meaning “to be / to become,” suggesting gradual change under heat — ripening, aging, or reaching maturity.

It covers physical ripeness and also skillful maturity — as in 熟練じゅくれん or 熟成じゅくせい.

この果物くだものは よく熟うれていて、おいしいです。
This fruit is nicely ripened and delicious.

彼かれは 熟練じゅくれんの 職人しょくにんです。
He is a skilled craftsman.