That your heart be not deceived; literally, lest your heart be enticed or seduced ( יִפְתָה). The verb means primarily to be open, and as a mind open to impressions from without is easily persuaded, moved either to good or evil, the word came to signify to induce in a good sense, or to seduce in a bad sense.
Here the people are cautioned against allowing themselves to be enticed so as to be led astray by seductive representations (cf. Job 31:27; Proverbs 20:19 ["flattereth"]; Job 5:2 ["silly one"]; Hosea 7:11).