'Thin' in the Bible

The one on whom seed was sown on rocky ground, this is the one who hears the word and at once welcomes it with joy;

Yet you do not know [the least thing] about what may happen in your life tomorrow. [What is secure in your life?] You are merely a vapor [like a puff of smoke or a wisp of steam from a cooking pot] that is visible for a little while and then vanishes [into thin air].