To stick to, to be persevering in holding to; as, to stick to a party or cause. ``The advantage will be on our side if we stick to its essentials.'' --Addison.

To stick up, to stand erect; as, his hair sticks up.

To stick up for, to assert and defend; as, to stick up for one's rights or for a friend. [Colloq.]

To stick upon, to dwell upon; not to forsake. ``If the matter be knotty, the mind must stop and buckle to it, and stick upon it with labor and thought.'' --Locke.

Source: Webster's Revised Unabridged Dictionary (1913)