What is Swear?

Legal Definition
To take an oath, judicially administered. Vide Affirmation; Oath.

2. To swear also signifies to use such profane language as is forbidden by law. This is generally punished by statutory provisions in the several states.
-- Bouviers Law Dictionary
Legal Definition
1. To put on oath ; to administer an oath to a person.

2. To take an oath; to become bound by an oath duly administered.

3. To use profane language. Swearing, in this sense, is made a punishable offence in many jurisdictions.
-- Black's Law Dictionary