Adam Clarke on Leviticus 20:9

Curseth his father or his mother - See the notes on Gen 48:12, and Exo 20:12 (note). He who conscientiously keeps the fifth commandment can be in no danger of this judgment. The term יקלל yekallel signifies, not only to curse, but to speak of a person contemptuously and disrespectfully, to make light of; so that all speeches which have a tendency to lessen our parents in the eyes of others, or to render their judgment, piety, etc., suspected and contemptible, may be here included; though the act of cursing, or of treating the parent with injurious and opprobrious language, is that which is particularly intended.